Patent Publication Number: US-2015089161-A1

Title: High performance interconnect coherence protocol

Description:
FIELD 
     The present disclosure relates in general to the field of computer development, and more specifically, to software development involving coordination of mutually-dependent constrained systems. 
     BACKGROUND 
     Advances in semi-conductor processing and logic design have permitted an increase in the amount of logic that may be present on integrated circuit devices. As a corollary, computer system configurations have evolved from a single or multiple integrated circuits in a system to multiple cores, multiple hardware threads, and multiple logical processors present on individual integrated circuits, as well as other interfaces integrated within such processors. A processor or integrated circuit typically comprises a single physical processor die, where the processor die may include any number of cores, hardware threads, logical processors, interfaces, memory, controller hubs, etc. 
     As a result of the greater ability to fit more processing power in smaller packages, smaller computing devices have increased in popularity. Smartphones, tablets, ultrathin notebooks, and other user equipment have grown exponentially. However, these smaller devices are reliant on servers both for data storage and complex processing that exceeds the form factor. Consequently, the demand in the high-performance computing market (i.e. server space) has also increased. For instance, in modern servers, there is typically not only a single processor with multiple cores, but also multiple physical processors (also referred to as multiple sockets) to increase the computing power. But as the processing power grows along with the number of devices in a computing system, the communication between sockets and other devices becomes more critical. 
     In fact, interconnects have grown from more traditional multi-drop buses that primarily handled electrical communications to full blown interconnect architectures that facilitate fast communication. Unfortunately, as the demand for future processors to consume at even higher-rates corresponding demand is placed on the capabilities of existing interconnect architectures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a simplified block diagram of a system including a point-to-point interconnect to connect I/O devices in a computer system in accordance with one embodiment; 
         FIG. 2  illustrates a simplified block diagram of a layered protocol stack in accordance with one embodiment; 
         FIG. 3  illustrates an embodiment of a transaction descriptor. 
         FIG. 4  illustrates an embodiment of a serial point-to-point link. 
         FIG. 5  illustrates embodiments of potential High Performance Interconnect (HPI) system configurations. 
         FIG. 6  illustrates an embodiment of a layered protocol stack associated with HPI. 
         FIG. 7  illustrates a flow diagram of example coherence protocol conflict management. 
         FIG. 8  illustrates a flow diagram of another example coherence protocol conflict management. 
         FIG. 9  illustrates a flow diagram of another example coherence protocol conflict management. 
         FIG. 10  illustrates an embodiment of a block diagram for a computing system including a multicore processor. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth, such as examples of specific types of processors and system configurations, specific hardware structures, specific architectural and micro architectural details, specific register configurations, specific instruction types, specific system components, specific processor pipeline stages, specific interconnect layers, specific packet/transaction configurations, specific transaction names, specific protocol exchanges, specific link widths, specific implementations, and operation etc. in order to provide a thorough understanding of the present invention. It may be apparent, however, to one skilled in the art that these specific details need not necessarily be employed to practice the subject matter of the present disclosure. In other instances, well detailed description of known components or methods has been avoided, such as specific and alternative processor architectures, specific logic circuits/code for described algorithms, specific firmware code, low-level interconnect operation, specific logic configurations, specific manufacturing techniques and materials, specific compiler implementations, specific expression of algorithms in code, specific power down and gating techniques/logic and other specific operational details of computer system in order to avoid unnecessarily obscuring the present disclosure. 
     Although the following embodiments may be described with reference to energy conservation, energy efficiency, processing efficiency, and so on in specific integrated circuits, such as in computing platforms or microprocessors, other embodiments are applicable to other types of integrated circuits and logic devices. Similar techniques and teachings of embodiments described herein may be applied to other types of circuits or semiconductor devices that may also benefit from such features. For example, the disclosed embodiments are not limited to server computer system, desktop computer systems, laptops, Ultrabooks™, but may be also used in other devices, such as handheld devices, smartphones, tablets, other thin notebooks, systems on a chip (SOC) devices, and embedded applications. Some examples of handheld devices include cellular phones, Internet protocol devices, digital cameras, personal digital assistants (PDAs), and handheld PCs. Here, similar techniques for a high-performance interconnect may be applied to increase performance (or even save power) in a low power interconnect. Embedded applications typically include a microcontroller, a digital signal processor (DSP), a system on a chip, network computers (NetPC), set-top boxes, network hubs, wide area network (WAN) switches, or any other system that can perform the functions and operations taught below. Moreover, the apparatus&#39;, methods, and systems described herein are not limited to physical computing devices, but may also relate to software optimizations for energy conservation and efficiency. As may become readily apparent in the description below, the embodiments of methods, apparatus&#39;, and systems described herein (whether in reference to hardware, firmware, software, or a combination thereof) may be considered vital to a “green technology” future balanced with performance considerations. 
     As computing systems are advancing, the components therein are becoming more complex. The interconnect architecture to couple and communicate between the components has also increased in complexity to ensure bandwidth demand is met for optimal component operation. Furthermore, different market segments demand different aspects of interconnect architectures to suit the respective market. For example, servers require higher performance, while the mobile ecosystem is sometimes able to sacrifice overall performance for power savings. Yet, it is a singular purpose of most fabrics to provide highest possible performance with maximum power saving. Further, a variety of different interconnects can potentially benefit from subject matter described herein. For instance, the Peripheral Component Interconnect (PCI) Express (PCIe) interconnect fabric architecture and QuickPath Interconnect (QPI) fabric architecture, among other examples, can potentially be improved according to one or more principles described herein, among other examples. 
       FIG. 1  illustrates one embodiment of a fabric composed of point-to-point Links that interconnect a set of components is illustrated. System  100  includes processor  105  and system memory  110  coupled to controller hub  115 . Processor  105  can include any processing element, such as a microprocessor, a host processor, an embedded processor, a co-processor, or other processor. Processor  105  is coupled to controller hub  115  through front-side bus (FSB)  106 . In one embodiment, FSB  106  is a serial point-to-point interconnect as described below. In another embodiment, link  106  includes a serial, differential interconnect architecture that is compliant with different interconnect standard. 
     System memory  110  includes any memory device, such as random access memory (RAM), non-volatile (NV) memory, or other memory accessible by devices in system  100 . System memory  110  is coupled to controller hub  115  through memory interface  116 . Examples of a memory interface include a double-data rate (DDR) memory interface, a dual-channel DDR memory interface, and a dynamic RAM (DRAM) memory interface. 
     In one embodiment, controller hub  115  can include a root hub, root complex, or root controller, such as in a PCIe interconnection hierarchy. Examples of controller hub  115  include a chipset, a memory controller hub (MCH), a northbridge, an interconnect controller hub (ICH) a southbridge, and a root controller/hub. Often the term chipset refers to two physically separate controller hubs, e.g., a memory controller hub (MCH) coupled to an interconnect controller hub (ICH). Note that current systems often include the MCH integrated with processor  105 , while controller  115  is to communicate with I/O devices, in a similar manner as described below. In some embodiments, peer-to-peer routing is optionally supported through root complex  115 . 
     Here, controller hub  115  is coupled to switch/bridge  120  through serial link  119 . Input/output modules  117  and  121 , which may also be referred to as interfaces/ports  117  and  121 , can include/implement a layered protocol stack to provide communication between controller hub  115  and switch  120 . In one embodiment, multiple devices are capable of being coupled to switch  120 . 
     Switch/bridge  120  routes packets/messages from device  125  upstream, i.e. up a hierarchy towards a root complex, to controller hub  115  and downstream, i.e. down a hierarchy away from a root controller, from processor  105  or system memory  110  to device  125 . Switch  120 , in one embodiment, is referred to as a logical assembly of multiple virtual PCI-to-PCI bridge devices. Device  125  includes any internal or external device or component to be coupled to an electronic system, such as an I/O device, a Network Interface Controller (NIC), an add-in card, an audio processor, a network processor, a hard-drive, a storage device, a CD/DVD ROM, a monitor, a printer, a mouse, a keyboard, a router, a portable storage device, a Firewire device, a Universal Serial Bus (USB) device, a scanner, and other input/output devices. Often in the PCIe vernacular, such as device, is referred to as an endpoint. Although not specifically shown, device  125  may include a bridge (e.g., a PCIe to PCI/PCI-X bridge) to support legacy or other versions of devices or interconnect fabrics supported by such devices. 
     Graphics accelerator  130  can also be coupled to controller hub  115  through serial link  132 . In one embodiment, graphics accelerator  130  is coupled to an MCH, which is coupled to an ICH. Switch  120 , and accordingly I/O device  125 , is then coupled to the ICH. I/O modules  131  and  118  are also to implement a layered protocol stack and associated logic to communicate between graphics accelerator  130  and controller hub  115 . Similar to the MCH discussion above, a graphics controller or the graphics accelerator  130  itself may be integrated in processor  105 . 
     Turning to  FIG. 2  an embodiment of a layered protocol stack is illustrated. Layered protocol stack  200  can includes any form of a layered communication stack, such as a QPI stack, a PCIe stack, a next generation high performance computing interconnect (HPI) stack, or other layered stack. In one embodiment, protocol stack  200  can include transaction layer  205 , link layer  210 , and physical layer  220 . An interface, such as interfaces  117 ,  118 ,  121 ,  122 ,  126 , and  131  in  FIG. 1 , may be represented as communication protocol stack  200 . Representation as a communication protocol stack may also be referred to as a module or interface implementing/including a protocol stack. 
     Packets can be used to communicate information between components. Packets can be formed in the Transaction Layer  205  and Data Link Layer  210  to carry the information from the transmitting component to the receiving component. As the transmitted packets flow through the other layers, they are extended with additional information used to handle packets at those layers. At the receiving side the reverse process occurs and packets get transformed from their Physical Layer  220  representation to the Data Link Layer  210  representation and finally (for Transaction Layer Packets) to the form that can be processed by the Transaction Layer  205  of the receiving device. 
     In one embodiment, transaction layer  205  can provide an interface between a device&#39;s processing core and the interconnect architecture, such as Data Link Layer  210  and Physical Layer  220 . In this regard, a primary responsibility of the transaction layer  205  can include the assembly and disassembly of packets (i.e., transaction layer packets, or TLPs). The translation layer  205  can also manage credit-based flow control for TLPs. In some implementations, split transactions can be utilized, i.e., transactions with request and response separated by time, allowing a link to carry other traffic while the target device gathers data for the response, among other examples. 
     Credit-based flow control can be used to realize virtual channels and networks utilizing the interconnect fabric. In one example, a device can advertise an initial amount of credits for each of the receive buffers in Transaction Layer  205 . An external device at the opposite end of the link, such as controller hub  115  in  FIG. 1 , can count the number of credits consumed by each TLP. A transaction may be transmitted if the transaction does not exceed a credit limit. Upon receiving a response an amount of credit is restored. One example of an advantage of such a credit scheme is that the latency of credit return does not affect performance, provided that the credit limit is not encountered, among other potential advantages. 
     In one embodiment, four transaction address spaces can include a configuration address space, a memory address space, an input/output address space, and a message address space. Memory space transactions include one or more of read requests and write requests to transfer data to/from a memory-mapped location. In one embodiment, memory space transactions are capable of using two different address formats, e.g., a short address format, such as a 32-bit address, or a long address format, such as 64-bit address. Configuration space transactions can be used to access configuration space of various devices connected to the interconnect. Transactions to the configuration space can include read requests and write requests. Message space transactions (or, simply messages) can also be defined to support in-band communication between interconnect agents. Therefore, in one example embodiment, transaction layer  205  can assemble packet header/payload  206 . 
     Quickly referring to  FIG. 3 , an example embodiment of a transaction layer packet descriptor is illustrated. In one embodiment, transaction descriptor  300  can be a mechanism for carrying transaction information. In this regard, transaction descriptor  300  supports identification of transactions in a system. Other potential uses include tracking modifications of default transaction ordering and association of transaction with channels. For instance, transaction descriptor  300  can include global identifier field  302 , attributes field  304  and channel identifier field  306 . In the illustrated example, global identifier field  302  is depicted comprising local transaction identifier field  308  and source identifier field  310 . In one embodiment, global transaction identifier  302  is unique for all outstanding requests. 
     According to one implementation, local transaction identifier field  308  is a field generated by a requesting agent, and can be unique for all outstanding requests that require a completion for that requesting agent. Furthermore, in this example, source identifier  310  uniquely identifies the requestor agent within an interconnect hierarchy. Accordingly, together with source ID  310 , local transaction identifier  308  field provides global identification of a transaction within a hierarchy domain. 
     Attributes field  304  specifies characteristics and relationships of the transaction. In this regard, attributes field  304  is potentially used to provide additional information that allows modification of the default handling of transactions. In one embodiment, attributes field  304  includes priority field  312 , reserved field  314 , ordering field  316 , and no-snoop field  318 . Here, priority sub-field  312  may be modified by an initiator to assign a priority to the transaction. Reserved attribute field  314  is left reserved for future, or vendor-defined usage. Possible usage models using priority or security attributes may be implemented using the reserved attribute field. 
     In this example, ordering attribute field  316  is used to supply optional information conveying the type of ordering that may modify default ordering rules. According to one example implementation, an ordering attribute of “0” denotes default ordering rules are to apply, wherein an ordering attribute of “1” denotes relaxed ordering, wherein writes can pass writes in the same direction, and read completions can pass writes in the same direction. Snoop attribute field  318  is utilized to determine if transactions are snooped. As shown, channel ID Field  306  identifies a channel that a transaction is associated with. 
     Returning to the discussion of  FIG. 2 , a Link layer  210 , also referred to as data link layer  210 , can act as an intermediate stage between transaction layer  205  and the physical layer  220 . In one embodiment, a responsibility of the data link layer  210  is providing a reliable mechanism for exchanging Transaction Layer Packets (TLPs) between two components on a link. One side of the Data Link Layer  210  accepts TLPs assembled by the Transaction Layer  205 , applies packet sequence identifier  211 , i.e. an identification number or packet number, calculates and applies an error detection code, i.e. CRC  212 , and submits the modified TLPs to the Physical Layer  220  for transmission across a physical to an external device. 
     In one example, physical layer  220  includes logical sub block  221  and electrical sub-block  222  to physically transmit a packet to an external device. Here, logical sub-block  221  is responsible for the “digital” functions of Physical Layer  221 . In this regard, the logical sub-block can include a transmit section to prepare outgoing information for transmission by physical sub-block  222 , and a receiver section to identify and prepare received information before passing it to the Link Layer  210 . 
     Physical block  222  includes a transmitter and a receiver. The transmitter is supplied by logical sub-block  221  with symbols, which the transmitter serializes and transmits onto to an external device. The receiver is supplied with serialized symbols from an external device and transforms the received signals into a bit-stream. The bit-stream is de-serialized and supplied to logical sub-block  221 . In one example embodiment, an 8b/10b transmission code is employed, where ten-bit symbols are transmitted/received. Here, special symbols are used to frame a packet with frames  223 . In addition, in one example, the receiver also provides a symbol clock recovered from the incoming serial stream. 
     As stated above, although transaction layer  205 , link layer  210 , and physical layer  220  are discussed in reference to a specific embodiment of a protocol stack (such as a PCIe protocol stack), a layered protocol stack is not so limited. In fact, any layered protocol may be included/implemented and adopt features discussed herein. As an example, a port/interface that is represented as a layered protocol can include: (1) a first layer to assemble packets, i.e. a transaction layer; a second layer to sequence packets, i.e. a link layer; and a third layer to transmit the packets, i.e. a physical layer. As a specific example, a high performance interconnect layered protocol, as described herein, is utilized. 
     Referring next to  FIG. 4 , an example embodiment of a serial point to point fabric is illustrated. A serial point-to-point link can include any transmission path for transmitting serial data. In the embodiment shown, a link can include two, low-voltage, differentially driven signal pairs: a transmit pair  406 / 411  and a receive pair  412 / 407 . Accordingly, device  405  includes transmission logic  406  to transmit data to device  410  and receiving logic  407  to receive data from device  410 . In other words, two transmitting paths, i.e. paths  416  and  417 , and two receiving paths, i.e. paths  418  and  419 , are included in some implementations of a link. 
     A transmission path refers to any path for transmitting data, such as a transmission line, a copper line, an optical line, a wireless communication channel, an infrared communication link, or other communication path. A connection between two devices, such as device  405  and device  410 , is referred to as a link, such as link  415 . A link may support one lane—each lane representing a set of differential signal pairs (one pair for transmission, one pair for reception). To scale bandwidth, a link may aggregate multiple lanes denoted by xN, where N is any supported link width, such as 1, 2, 4, 8, 12, 16, 32, 64, or wider. 
     A differential pair can refer to two transmission paths, such as lines  416  and  417 , to transmit differential signals. As an example, when line  416  toggles from a low voltage level to a high voltage level, i.e. a rising edge, line  417  drives from a high logic level to a low logic level, i.e. a falling edge. Differential signals potentially demonstrate better electrical characteristics, such as better signal integrity, i.e. cross-coupling, voltage overshoot/undershoot, ringing, among other example advantages. This allows for a better timing window, which enables faster transmission frequencies. 
     In one embodiment, a new High Performance Interconnect (HPI) is provided. HPI can include a next-generation cache-coherent, link-based interconnect. As one example, HPI may be utilized in high performance computing platforms, such as workstations or servers, including in systems where PCIe or another interconnect protocol is typically used to connect processors, accelerators, I/O devices, and the like. However, HPI is not so limited. Instead, HPI may be utilized in any of the systems or platforms described herein. Furthermore, the individual ideas developed may be applied to other interconnects and platforms, such as PCIe, MIPI, QPI, etc. 
     To support multiple devices, in one example implementation, HPI can include an Instruction Set Architecture (ISA) agnostic (i.e. HPI is able to be implemented in multiple different devices). In another scenario, HPI may also be utilized to connect high performance I/O devices, not just processors or accelerators. For example, a high performance PCIe device may be coupled to HPI through an appropriate translation bridge (i.e. HPI to PCIe). Moreover, the HPI links may be utilized by many HPI based devices, such as processors, in various ways (e.g. stars, rings, meshes, etc.).  FIG. 5  illustrates example implementations of multiple potential multi-socket configurations. A two-socket configuration  505 , as depicted, can include two HPI links; however, in other implementations, one HPI link may be utilized. For larger topologies, any configuration may be utilized as long as an identifier (ID) is assignable and there is some form of virtual path, among other additional or substitute features. As shown, in one example, a four socket configuration  510  has an HPI link from each processor to another. But in the eight socket implementation shown in configuration  515 , not every socket is directly connected to each other through an HPI link. However, if a virtual path or channel exists between the processors, the configuration is supported. A range of supported processors includes 2-32 in a native domain. Higher numbers of processors may be reached through use of multiple domains or other interconnects between node controllers, among other examples. 
     The HPI architecture includes a definition of a layered protocol architecture, including in some examples, protocol layers (coherent, non-coherent, and, optionally, other memory based protocols), a routing layer, a link layer, and a physical layer including associated I/O logic. Furthermore, HPI can further include enhancements related to power managers (such as power control units (PCUs)), design for test and debug (DFT), fault handling, registers, security, among other examples.  FIG. 6  illustrates an embodiment of an example HPI layered protocol stack. In some implementations, at least some of the layers illustrated in  FIG. 6  may be optional. Each layer deals with its own level of granularity or quantum of information (the protocol layer  605   a,b  with packets  630 , link layer  610   a,b  with flits  635 , and physical layer  605   a,b  with phits  640 ). Note that a packet, in some embodiments, may include partial flits, a single flit, or multiple flits based on the implementation. 
     As a first example, a width of a phit  640  includes a 1 to 1 mapping of link width to bits (e.g. 20 bit link width includes a phit of 20 bits, etc.). Flits may have a greater size, such as 184, 192, or 200 bits. Note that if phit  640  is 20 bits wide and the size of flit  635  is 184 bits then it takes a fractional number of phits  640  to transmit one flit  635  (e.g. 9.2 phits at 20 bits to transmit an 184 bit flit  635  or 9.6 at 20 bits to transmit a 192 bit flit, among other examples). Note that widths of the fundamental link at the physical layer may vary. For example, the number of lanes per direction may include 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, etc. In one embodiment, link layer  610   a,b  is capable of embedding multiple pieces of different transactions in a single flit, and one or multiple headers (e.g. 1, 2, 3, 4) may be embedded within the flit. In one example, HPI splits the headers into corresponding slots to enable multiple messages in the flit destined for different nodes. 
     Physical layer  605   a,b , in one embodiment, can be responsible for the fast transfer of information on the physical medium (electrical or optical etc.). The physical link can be point-to-point between two Link layer entities, such as layer  605   a  and  605   b . The Link layer  610   a,b  can abstract the Physical layer  605   a,b  from the upper layers and provides the capability to reliably transfer data (as well as requests) and manage flow control between two directly connected entities. The Link Layer can also be responsible for virtualizing the physical channel into multiple virtual channels and message classes. The Protocol layer  620   a,b  relies on the Link layer  610   a,b  to map protocol messages into the appropriate message classes and virtual channels before handing them to the Physical layer  605   a,b  for transfer across the physical links. Link layer  610   a,b  may support multiple messages, such as a request, snoop, response, writeback, non-coherent data, among other examples. 
     The Physical layer  605   a,b  (or PHY) of HPI can be implemented above the electrical layer (i.e. electrical conductors connecting two components) and below the link layer  610   a,b , as illustrated in  FIG. 6 . The Physical layer and corresponding logic can reside on each agent and connects the link layers on two agents (A and B) separated from each other (e.g. on devices on either side of a link). The local and remote electrical layers are connected by physical media (e.g. wires, conductors, optical, etc.). The Physical layer  605   a,b , in one embodiment, has two major phases, initialization and operation. During initialization, the connection is opaque to the link layer and signaling may involve a combination of timed states and handshake events. During operation, the connection is transparent to the link layer and signaling is at a speed, with all lanes operating together as a single link. During the operation phase, the Physical layer transports flits from agent A to agent B and from agent B to agent A. The connection is also referred to as a link and abstracts some physical aspects including media, width and speed from the link layers while exchanging flits and control/status of current configuration (e.g. width) with the link layer. The initialization phase includes minor phases e.g. Polling, Configuration. The operation phase also includes minor phases (e.g. link power management states). 
     In one embodiment, Link layer  610   a,b  can be implemented so as to provide reliable data transfer between two protocol or routing entities. The Link layer can abstract Physical layer  605   a,b  from the Protocol layer  620   a,b , and can be responsible for the flow control between two protocol agents (A, B), and provide virtual channel services to the Protocol layer (Message Classes) and Routing layer (Virtual Networks). The interface between the Protocol layer  620   a,b  and the Link Layer  610   a,b  can typically be at the packet level. In one embodiment, the smallest transfer unit at the Link Layer is referred to as a flit which a specified number of bits, such as 192 bits or some other denomination. The Link Layer  610   a,b  relies on the Physical layer  605   a,b  to frame the Physical layer&#39;s  605   a,b  unit of transfer (phit) into the Link Layer&#39;s  610   a,b  unit of transfer (flit). In addition, the Link Layer  610   a,b  may be logically broken into two parts, a sender and a receiver. A sender/receiver pair on one entity may be connected to a receiver/sender pair on another entity. Flow Control is often performed on both a flit and a packet basis. Error detection and correction is also potentially performed on a flit level basis. 
     In one embodiment, Routing layer  615   a,b  can provide a flexible and distributed method to route HPI transactions from a source to a destination. The scheme is flexible since routing algorithms for multiple topologies may be specified through programmable routing tables at each router (the programming in one embodiment is performed by firmware, software, or a combination thereof). The routing functionality may be distributed; the routing may be done through a series of routing steps, with each routing step being defined through a lookup of a table at either the source, intermediate, or destination routers. The lookup at a source may be used to inject a HPI packet into the HPI fabric. The lookup at an intermediate router may be used to route an HPI packet from an input port to an output port. The lookup at a destination port may be used to target the destination HPI protocol agent. Note that the Routing layer, in some implementations, can be thin since the routing tables, and, hence the routing algorithms, are not specifically defined by specification. This allows for flexibility and a variety of usage models, including flexible platform architectural topologies to be defined by the system implementation. The Routing layer  615   a,b  relies on the Link layer  610   a,b  for providing the use of up to three (or more) virtual networks (VNs)—in one example, two deadlock-free VNs, VN0 and VN1 with several message classes defined in each virtual network. A shared adaptive virtual network (VNA) may be defined in the Link layer, but this adaptive network may not be exposed directly in routing concepts, since each message class and virtual network may have dedicated resources and guaranteed forward progress, among other features and examples. 
     In one embodiment, HPI can include a Coherence Protocol layer  620   a,b  to support agents caching lines of data from memory. An agent wishing to cache memory data may use the coherence protocol to read the line of data to load into its cache. An agent wishing to modify a line of data in its cache may use the coherence protocol to acquire ownership of the line before modifying the data. After modifying a line, an agent may follow protocol requirements of keeping it in its cache until it either writes the line back to memory or includes the line in a response to an external request. Lastly, an agent may fulfill external requests to invalidate a line in its cache. The protocol ensures coherency of the data by dictating the rules all caching agents may follow. It also provides the means for agents without caches to coherently read and write memory data. 
     Two conditions may be enforced to support transactions utilizing the HPI Coherence Protocol. First, the protocol can maintain data consistency, as an example, on a per-address basis, among data in agents&#39; caches and between those data and the data in memory. Informally, data consistency may refer to each valid line of data in an agent&#39;s cache representing a most up-to-date value of the data and data transmitted in a coherence protocol packet can represent the most up-to-date value of the data at the time it was sent. When no valid copy of the data exists in caches or in transmission, the protocol may ensure the most up-to-date value of the data resides in memory. Second, the protocol can provide well-defined commitment points for requests. Commitment points for reads may indicate when the data is usable; and for writes they may indicate when the written data is globally observable and will be loaded by subsequent reads. The protocol may support these commitment points for both cacheable and uncacheable (UC) requests in the coherent memory space. 
     The HPI Coherence Protocol also may ensure the forward progress of coherence requests made by an agent to an address in the coherent memory space. Certainly, transactions may eventually be satisfied and retired for proper system operation. The HPI Coherence Protocol, in some embodiments, may have no notion of retry for resolving resource allocation conflicts. Thus, the protocol itself may be defined to contain no circular resource dependencies, and implementations may take care in their designs not to introduce dependencies that can result in deadlocks. Additionally, the protocol may indicate where designs are able to provide fair access to protocol resources. 
     Logically, the HPI Coherence Protocol, in one embodiment, can include three items: coherence (or caching) agents, home agents, and the HPI interconnect fabric connecting the agents. Coherence agents and home agents can work together to achieve data consistency by exchanging messages over the interconnect. The link layer  610   a,b  and its related description can provide the details of the interconnect fabric including how it adheres to the coherence protocol&#39;s requirements, discussed herein. (It may be noted that the division into coherence agents and home agents is for clarity. A design may contain multiple agents of both types within a socket or even combine agents behaviors into a single design unit, among other examples.) 
     In one embodiment, home agents can be configured to guard physical memory. Each home agent can be responsible for a region of the coherent memory space. Regions may be non-overlapping, in that a single address is guarded by one home agent, and together the home agent regions in a system cover the coherent memory space. For instance, each address can be guarded by at least one home agent. Therefore, in one embodiment, each address in a HPI system&#39;s coherent memory space can map to exactly one home agent. 
     Home agents in the HPI Coherence Protocol, in one embodiment, can be responsible for servicing requests to the coherent memory space. For read (Rd) requests, home agents may generate snoops (Snp), process their responses, send a data response, and send a completion response. For invalidation (Inv) requests, home agents may generate necessary snoops, process their responses, and send a completion response. For write requests, home agents may commit the data to memory and send a completion response. 
     Home agents may provide snoops in the HPI Coherence Protocol and process snoop responses from coherence agents. Home agents can also process forward requests, which are a special snoop response, from coherence agents for conflict resolution. When a home agent receives a forward request, it may send a forward response to the coherence agent that generated the forward request (i.e., the agent that detected a conflicting snoop request). Coherence agents can use the ordering of these forward responses and completion responses from the home agent to resolve conflicts. 
     A coherence agent may issue supported coherence protocol requests. Requests may be issued to an address in the coherent memory space. Data received for read requests (Rd) except RdCur may be consistent. Data for RdCur requests may have been consistent when the data packet was generated (although it may have become out of date during delivery). Table 1 shows an exemplary, non-exhaustive list of potential supported requests: 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Name 
                 Semantics 
                 Cache State 
               
               
                   
               
             
            
               
                 RdCode 
                 Request a cache line in F or S state. 
                 F or S 
               
               
                 RdData 
                 Request a cache line in E, F, or S state. 
                 F or S 
               
               
                 RdMigr 
                 Request a cache line in M, E, F, or S state. 
                 M and (F or S) 
               
               
                 RdInv 
                 Request a cache line in E state. If line was previously 
                 E 
               
               
                   
                 cached in M state, the line will be written to memory 
               
               
                   
                 before E data is delivered. 
               
               
                 RdInvOwn 
                 Request a cache line in M or E state. 
                 M 
               
               
                 RdCur 
                 Request an uncacheable snapshot of a cache line. 
               
               
                 InvItoE 
                 Request exclusive ownership of a cache line without 
                 M or E 
               
               
                   
                 receiving data. 
               
               
                 InvItoM 
                 Request exclusive ownership of a cache line without 
                 M or E 
               
               
                   
                 receiving data and with the intent of performing a 
               
               
                   
                 writeback soon afterward. 
               
               
                 InvXtoI 
                 Flush a cache line from all caches. Requesting agent is to 
               
               
                   
                 invalidate the line in its cache before issuing this request. 
               
               
                 WbMtoI 
                 Write a cache line in M state back to memory and 
                 M 
               
               
                   
                 invalidate the line in the cache. 
               
               
                 WbMtoS 
                 Write a cache line in M state back to memory and 
                 M and S 
               
               
                   
                 transition line to S state. 
               
               
                 WbMtoE 
                 Write a cache line in M state back to memory and 
                 M and E 
               
               
                   
                 transition line to E state. 
               
               
                 WbMtoIPtl 
                 Write a cache line in M state back to memory, according 
                 M 
               
               
                   
                 to a byte-enable mask, and transition line to I state. 
               
               
                 WbMtoEPtl 
                 Write a cache line in M state back to memory, according 
                 M and E 
               
               
                   
                 to a byte-enable mask, transition line to E state, and clear 
               
               
                   
                 the line&#39;s mask in the cache. 
               
               
                 EvctCln 
                 Notification to home agent that a cache line in E state was 
                 E 
               
               
                   
                 invalidated in the cache. 
               
               
                 WbPushMtoI 
                 Send a line in M state to home agent and invalidate the 
                 M 
               
               
                   
                 line in the cache; home agent may either write the line 
               
               
                   
                 back to memory or send it to a local cache agent with M 
               
               
                   
                 state. 
               
               
                 WbFlush 
                 Request that home flush writes to implementation- 
               
               
                   
                 specific addresses in its memory hierarchy. No data is 
               
               
                   
                 sent with the request. 
               
               
                   
               
            
           
         
       
     
     HPI can support a Coherency protocol making use of principles of the MESI protocol. Each cache line can be marked with one or more supported states (e.g., coded in the cache line). A “M” or “Modified” state can indicate that the cache line value has been modified from that value which is in main memory. A line in the M-state is only present in the particular and the corresponding cache agent can be required to write the modified data back to memory at some time in the future, for instance, before permitting any other read of the (no longer valid) maing memory state. A writeback can transition the line from the M-state to the E-state. The “E” or “Exclusive” state can indicate that the cache line is only present in the current cache but that its value matches that in main memory. The cache line in E-state can transition to the S-state at any time in response to a read request or may be changed to the M-state by writing to the line. The “S” or “Shared” state can indicates that the cache line may be stored in other caches of the machine and has a value that matches that of the main memory. The line may be discarded (changed to the I-state) at any time. The “I” or “Invalid” state can indicate that a cache line is invalid or unused. Other state can also supported in HPI, such as an “F” or “Forward” shared state that indicates that the particular shared line value is to be forwarded to other caches that are to also share the line, among other examples. 
     Table 2 include exemplary information that can be included in some Coherence protocol messages, including snoop, read, and write requests, among other examples: 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Field 
                 Usage 
               
               
                   
               
             
            
               
                 cmd 
                 Message command (or name or opcode). 
               
               
                 addr 
                 Address of a coherent cache line. 
               
               
                 destNID 
                 Node ID (NID) of destination (home or coherence) agent. 
               
               
                 reqNID 
                 NID of requesting coherence agent. 
               
               
                 peerNID 
                 NID of coherence agent that sent the (forward request) 
               
               
                   
                 message. 
               
               
                 reqTID 
                 ID of the resource allocated by the requesting agent for the 
               
               
                   
                 transaction, also known as RTID (or requesting transaction 
               
               
                   
                 identifier). 
               
               
                 homeTID 
                 ID of the resource allocated by the home agent to process the 
               
               
                   
                 request, also known as HTID (or home transaction identifier). 
               
               
                 data 
                 A cache line of data. 
               
               
                 mask 
                 Byte mask to qualify the data. 
               
               
                   
               
            
           
         
       
     
     Snoop messages may be generated by home agents and directed toward coherence agents. A snoop (SNP) virtual channel can be used for snoops and, in one embodiment, are the only messages that use the SNP virtual channel. Snoops can include the requesting agent&#39;s NID and the RTID it allocated for the request in case the snoop results in data being sent directly to the requesting agent. Snoops, in one embodiment, can also include the HTID allocated by the home agent to process the request. The coherence agent processing the snoop may include the HTID in the snoop response it sends back to the home agent. Snoops may, in some instance, not include the home agent&#39;s NID because it may be derived from the included address, which the targeted coherence agent does when sending its response. Fanout snoops (those with “SnpF” prefix) may not include a destination NID because the Routing Layer is responsible for generating the appropriate snoop messages to all peers in the fanout region. An exemplary list of snoop channel messages is listed Table 3: 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Command 
                 Semantics 
                 Fields 
               
               
                   
               
             
            
               
                 SnpCode 
                 Snoop to get data in F or S state. 
                 cmd, 
               
               
                 SnpData 
                 Snoop to get data in E, F, or S state. 
                 addr, 
               
               
                 SnpMigr 
                 Snoop to get data in M, E, F, or S state. 
                 destNID, 
               
               
                 SnpInv 
                 Snoop to invalidate the peer&#39;s cache, flushing any M copy to 
                 reqNID, 
               
               
                   
                 memory. 
                 reqTID, 
               
               
                 SnpInvOwn 
                 Snoop to get data in M or E state. 
                 homeTID 
               
               
                 SnpCur 
                 Snoop to get an uncacheable snapshot of a cache line. 
               
               
                 SnpFCode 
                 Snoop to get data in F or S state; Routing layer to handle 
                 cmd, 
               
               
                   
                 distribution to all fanout peers 
                 addr, 
               
               
                 SnpFData 
                 Snoop to get data in E, F, or S state; Routing layer to handle 
                 reqNID, 
               
               
                   
                 distribution to all fanout peers 
                 reqTID, 
               
               
                 SnpFMigr 
                 Snoop to get data in M, E, F, or S state; Routing layer to 
                 homeTID 
               
               
                   
                 handle distribution to all fanout peers 
               
               
                 SnpFInvOwn 
                 Snoop to get data in M or E state: Routing layer to handle 
               
               
                   
                 distribution to all fanout peers. 
               
               
                 SnpFInv 
                 Snoop to invalidate the peer&#39;s cache, flushing any M copy to 
               
               
                   
                 memory; Routing layer to handle distribution to all fanout 
               
               
                   
                 peers. 
               
               
                 SnpCur 
                 Snoop to get an uncacheable snapshot of a, cache line: Routing 
               
               
                   
                 layer to handle distribution to all fanout peers. 
               
               
                   
               
            
           
         
       
     
     HPI may also support non snoop requests that they may issue to an address, such as those implemented as non-coherent requests. Examples of such requests can include a non-snoop read to request a read-only line form memory, a non-snoop write to write a line to memory, and a write a line to memory according to a mask, among other potential examples. 
     In one example, four general types of response messages can be defined in the HPI Coherence Protocol: data, completion, snoop, and forward. Certain data messages can carry an additional completion indication and certain snoop responses can carry data. Response messages may use the RSP virtual channel, and the communication fabric may maintain proper message delivery ordering among ordered completion responses and forward responses. 
     Table 4 includes a listing of at least some potential response messages supported by an example HPI Coherence Protocol: 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Name 
                 Semantics 
                 Fields 
               
               
                   
               
             
            
               
                 Data_M 
                 Data is M state. 
                 cmd, 
               
               
                 Data_E 
                 Data is E state. 
                 destNID, 
               
               
                 Data_F 
                 Data is F state. 
                 reqTID, 
               
               
                 Data_SI 
                 Depending upon request, data in S state or uncacheable 
                 data 
               
               
                   
                 “snapshot” data. 
               
               
                 Data_M 
                 Data is M state with an ordered completion response. 
               
               
                 Data_E 
                 Data is E state with an ordered completion response. 
               
               
                 Data_F 
                 Data is F state with an ordered completion response. 
               
               
                 Data_SI 
                 Depending upon request, data in S state or uncacheable 
               
               
                   
                 “snapshot” data, with an ordered completion response. 
               
               
                 CmpU 
                 Completion message with no ordering requirements. 
                 cmd, 
               
               
                 CmpO 
                 Completion message to be ordered with forward 
                 destNID, 
               
               
                   
                 responses. 
                 reqTID 
               
               
                 RspI 
                 Cache is in I state. 
                 cmd, 
               
               
                 RspS 
                 Cache is in S state. 
                 destNID, 
               
               
                 RspFwd 
                 Copy of cache line was sent to requesting agent, cache 
                 homeTID 
               
               
                   
                 state did not change. 
               
               
                 RspFwdI 
                 Copy of cache line was sent to requesting agent, cache 
               
               
                   
                 transitions to I state. 
               
               
                 RspFwdS 
                 Copy of cache line was sent to requesting agent, cache 
               
               
                   
                 transitions to S state. 
               
               
                 RspIWb 
                 Modified line is being implicitly written back to memory, 
                 cmd, 
               
               
                   
                 cache was transitioned to I state. 
                 destNID, 
               
               
                 RspSWb 
                 Modified line is being implicitly written back to memory, 
                 homeTID, 
               
               
                   
                 cache was transitioned to S state. 
                 data 
               
               
                 RspFwdIWb 
                 Modified line is being implicitly written back to memory, 
               
               
                   
                 copy of cache line was sent to requesting agent, cache 
               
               
                   
                 was transitioned to I state. 
               
               
                 RspFwdSWb 
                 Modified line is being implicitly written back to memory, 
               
               
                   
                 copy of cache line was sent to requesting agent, cache 
               
               
                   
                 was transitioned to S state. 
               
               
                 RspCnflt 
                 Peer has an outstanding request to same address, is 
                 cmd, 
               
               
                   
                 requesting an ordered forward response, and has 
                 destNID, 
               
               
                   
                 allocated a resource for the forward. 
                 homeTID, 
               
               
                   
                   
                 peerNID 
               
               
                   
               
            
           
         
       
     
     In one example, data responses can target a requesting coherence agent. A home agent may send any of the data responses. A coherence agent may send only data responses not containing an ordered completion indication. Additionally, coherence agents may be limited to sending data responses only as a result of processing a snoop request. Combined data and completion responses may always be of the ordered-completion type and can be kept ordered with forward responses by the communication fabric. 
     The HPI Coherence Protocol can uses the general unordered completion message and a coherence-specific ordered completion message. A home agent may send completion responses to coherent requests and completion responses can be typically destined for a coherence agent. The ordered completion response can be kept ordered with forward responses by the communication fabric. 
     Snoop responses may be sent by coherence agents, specifically in response to processing a snoop request, and target the home agent handling the snoop request. The destNID is usually a home agent (determined from the address in the snoop request) and the included TID is for the home agent&#39;s resource allocated to process the request. Snoop responses with “Wb” in the command are for implicit writebacks of modified cache lines, and they carry the cache line data. (Implicit writebacks can include those a coherence agent makes due to another agent&#39;s request, whereas the other requests are made explicitly by the coherence agent using its request resources.) 
     Coherence agents can generate a forward request when a snoop request conflicts with an outstanding request. Forward requests target the home agent that generated the snoop, which is determined from the address in the snoop request. Thus, the destNID is a home agent. The forward request can also include the TID for the home agent&#39;s resource allocated to process the original request and the NID of the coherence agent generating the forward request 
     The HPI Coherence Protocol can support a single forward response, FwdCnfltO. Home agents can send a forward response for every forward request received and to the coherence agent in the forward request&#39;s peerNID field. Forward responses carry the cache line address so the coherence agent can match the message to the forward resource it allocated. Forward response message can carry the requesting agent&#39;s NID but, in some cases, not the requesting agent&#39;s TID. If a coherence agent wants to support cache-to-cache transfers for forward responses, it can save the requesting agent&#39;s TID when processing the snoop and send a forward request. To support conflict resolution, the communication fabric may maintain ordering between the forward response and all ordered completions sent before it to the same destination coherence agent. 
     In some systems, home agent resources are pre-allocated in that “RTIDs” represent resources in the home agents and the caching agents allocate RTIDs from system-configured pools when generating new coherence requests. Such schemes can limit the number of active requests any particular caching agent can have to a home agent to the number of RTIDs it was given by the system, effectively slicing up home resources statically among caching agents. Such schemes can result inefficient allocation of resources and properly sizing a home agent to support request throughput can become impractical for large systems, among other potential issues. For instance, such schemes can force RTID pool management upon the caching agents. Additionally, in some systems, a caching agent may not reuse the RTID until the home agent has completely processed the transaction. Waiting until a home agent completes all processing, however, can unnecessarily throttle caching agents. Additionally, certain flows in the protocol can involve caching agents holding onto RTIDs beyond the home agent release notification, further throttling their performance, among other issues. 
     In one implementation, home agents can be allowed to allocate their resources as requests arrive from cache agents. In such instances, home agent resource management can be kept separate from coherence agent logic. In some implementations, home resource management and coherence agent logic can be at least partially intermingled. In some instances, coherence agents can have more outstanding requests to a home agent than the home agent can simultaneously handle. For instance, HPI can allow requests to queue up in the communication fabric. Further, to avoid deadlocks caused by the home agent blocking incoming requests until resources become available, the HPI Coherence protocol can be configured to ensure that other messages can make progress around blocked requests to ensure that active transactions reach completion. 
     In one example, resource management can be supported by allowing an agent receiving a request to allocate resources to process it, the agent sending the request allocating respective resources for all responses to the request The HTID can represent the resource that a home agent allocates for a given request included in some protocol messages. The HTID (along with RNID/RTID) in snoop requests and forward responses can be used to support responses to a home agent as well as data forwarding to a requesting agent, among other examples. Further, HPI can support the ability of an agent to send an ordered complete (CmpO) early, that is, before the home agent is finished processing the request, when it is determined to be safe for a requesting agent to reuse its RTID resource. General handling of snoops with similar RNID/RTID can also be defined by the protocol. 
     In one illustrative example, when a particular request&#39;s tracker state is busy, a directory state can be used to determine when the home agent may send a response. For instance, an Invalid directory state can allow a response to be sent, except for RdCur requests which indicates there are no outstanding snoop responses. An Unknown directory state can dictate that all peer agents have been snooped and all their responses gathered before a response can be sent. The Exclusive directory state can dictate that the owner be snooped and all responses gathered before a response is sent, or if the requesting agent is the owner then a response may immediately be sent. The Shared directory state can specify that an invalidating request (e.g., RdInv* or Inv*) has snooped all peer agents and gathered all snoop responses. When a given request&#39;s tracker state is writeback buffered (WbBuffered), the home agent may send a data response. When the request&#39;s tracker state is DataSent (indicating the home agent has already sent a data response) or DataXfrd (indicating a peer transferred a copy of the line), the home agent may send the completion response. 
     In instances such as those described above, a home agent may send data and completion responses before all snoop responses have been gathered. The HPI interface allows these “early” responses. When sending early data and completions, the home agent may gather all outstanding snoop responses before releasing the resource it allocated for the request. The home agent can also continue blocking further standard requests to the same address until all snoop responses have been gathered, then releasing the resource. A home agent sending a response message from a Busy or WbBuffered state can use a sub-action table (e.g., included in a set of protocol tables embodying the formal specification of the HPI Coherence protocol) for which message to send and use a sub action table for how to update the directory state, among other examples. In some cases, an early completion can be performed without pre-allocation by a home node. 
     In one embodiment, HPI Coherence protocol can omit the use of either or both pre-allocated home resources and ordered request channels. In such implementations, certain messages on the HPI RSP communication channel can be ordered. For instance, specifically “ordered completion” and “forward response” messages, can be provided, that can be sent from the home agent to the coherence agent. Home agents can send an ordered completion (CmpO or Data_*_CmpO) for all coherent read and invalidation requests (as well as other requests, such as a NonSnpRd requests, that are not involved in cache-coherence conflicts). 
     Home agents can send forward responses (FwdCnfltO) to coherence agents that send forward requests (RspCnflt) to indicate a conflict. A coherence agent can generate a forward request whenever it has an outstanding read or invalidation request and detects an incoming snoop request to the same cache line as the request. When the coherence agent receives the forward response, it checks the current state of the outstanding request to determine how to process the original snoop. The home agent can sent the forward response to be ordered with a complete (e.g., CmpO or Data_*_CmpO). The coherence agent can utilize information included in the snoop to aid the coherence agent in processing a forward response. For instance, a forward response may not include any “type” information and no RTID. The nature of the forward response can be derived from information obtained from the preceding snoop(s). Further, a coherence agent may block outstanding snoop requests when all of its “forward resources” are waiting for forward responses. In some implementations, each coherence agent can be designed to have at least one forward resource. 
     In some implementations, communication fabric requirements can be upon the Routing Layer. In one embodiment, the HPI Coherence protocol has one communication fabric requirement that is specific to the Routing Layer. The coherence protocol can depend upon the routing layer to convert a fanout snoop (SnpF* opcodes—Snoop (SNP) Channel Messages”) into the appropriate snoops for all of the request&#39;s peers in the fanout set of Coherence Agents. The fanout set is a configuration parameter of the Routing Layer that is shared by the Protocol Layer. In this coherence protocol specification it is described as a Home Agent configuration parameter. 
     In some implementations above, the HPI Coherence Protocol can utilizes four of the virtual channels: REQ, WB, SNP, and RSP. The virtual channels can be used to unwind dependency cycles and avoid deadlock. In one embodiment, every message can be delivered without duplication on all virtual channels and an ordering requirement upon the RSP virtual channel. 
     In some implementations, the communication fabric can be configured to preserve an ordering among certain completion messages and the FwdCnfltO message. The completion messages are the CmpO message and any data message with CmpO attached (Data_*_CmpO). Together, all of these messages are the “ordered completion responses.” The conceptual requirement between ordered completion responses and the FwdCnfltO message is that a FwdCnfltO does not “pass” an ordered completion. More specifically, if a home agent sends an ordered completion response followed by a FwdCnfltO message and both messages are destined for the same coherence agent, then the communication fabric delivers the ordered completion response before the FwdCnfltO, among other potential examples. 
     It should be appreciated that while some examples of the protocol flow are disclosed herein, the described examples are merely intended to give an intuitive feel for the protocol and do not necessarily cover all possible scenarios and behaviors the protocol may exhibit. 
     A conflict may occur when requests to the same cache-line address from more than one coherence agent occur around the same time. As a specific example, a conflict can occur when a snoop for a coherence agent&#39;s standard request arrives at a peer coherence agent with an outstanding request to the same address. Because each snoop may end up in a conflict, a single request can have multiple conflicts. Resolving conflicts may be a coordinated effort among the home agent, the coherence agents, and the communication fabric. However, the primary responsibility lies with the coherence agents detecting conflicting snoops. 
     In one embodiment, home agents, coherence agents, and communication fabric can be configured to assist in successfully resolving conflicts. For example, home agents may have outstanding snoops for only one request per address at a time, such that, for a given address, a home agent may have outstanding snoops for only one request. This can serve to exclude the possibility of race conditions involving two requests conflicting with each other. It can also ensure that a coherence agent will not see another snoop to the same address after it has detected a conflict but not yet resolved it. 
     In another example, when a coherence agent processes a snoop with an address matching an active standard request, it can allocates a forward resource and sends a forward request to the home agent. A coherence agent with an outstanding standard request that receives a snoop to the same address can responds with a RspCnflt snoop response. This response can be a forward request to the home agent. Because the message is a request, before sending it the coherence agent can allocate a resource to handle the response that the home agent will send. (The coherence protocol allows blocking conflicting snoops when the coherence agent has run out of forward resources, in some instances.) The coherence agent may store information about the conflicting snoop to use when processing the forward response. After detecting a conflict and until processing the forward response, a coherence agent may be guaranteed to not see another snoop to the same address. 
     In some examples, when a home agent receives a forward request, it does not record the snoop response. Instead, the home agent can send a forward response to the conflicting coherence agent. A forward request (RspCnflt), in one example, looks like a snoop response but the home agent does not treat it as one. It does not record the message as a snoop response, but instead sends a forward response. Specifically, for every forward request (RspCnflt) a home agent receives, it sends a forward response (FwdCnfltO) to the requesting coherence agent. 
     The HPI Communication Fabric orders forward responses and ordered completions between the home agent and the targeted coherence agent. The fabric can thereby serve to differentiate an early conflict from a late conflict at the conflicting coherence agent. From a system-level perspective, an early conflict occurs when a snoop encounters a request that the home agent has not yet processed, and a late conflict occurs when a snoop encounters a request that the home agent has already processed. From a home agent&#39;s perspective, an early conflict is when a snoop for the currently active request encounters a request that the home agent has not yet received or started processing, and a late conflict is when the snoop encounters a request it has already processed. In other words, a late conflict is with a request to which the home agent has already sent a completion response. Thus, when a home agent receives a forward request for a late conflict, it will have already sent the completion response to the conflicting agent&#39;s outstanding request. By ordering the forward responses and ordered completion responses from home agent to the coherence agent, the coherence agent can determine whether the conflict was early or late by the processing state of its conflicting request. 
     When a coherence agent receives a forward response, it uses the state of its conflicting request to determine whether the conflict was early or late and when to process the original snoop. Because of the communication fabric&#39;s ordering requirement, the state of the conflicting request indicates whether the conflict was early or late. If the request state indicates the completion has been received then it was a late conflict, otherwise it was an early conflict. Alternatively, if the request state indicates the request is still waiting for its response(s) then it was an early conflict, otherwise it was a late conflict. The type of conflict determines when to process the snoop: From a coherence agent&#39;s perspective, an early conflict means the snoop is for a request being processed before the agent&#39;s conflicting request, and a late conflict means the snoop is for a request being processed after the agent&#39;s conflicting request. Given that ordering, for an early conflict, the coherence agent immediately processes the original snoop; and for a late conflict, the coherence agent waits until the conflicting request has received its data (for reads) and its processor has had an opportunity to act upon the finished request before processing the snoop. When the conflicting snoop is processed, the coherence agent will generate a snoop response for the home agent to finally record. 
     All conflicts with writeback requests can be late conflicts. A late conflict from the coherence agent&#39;s perspective is when the agent&#39;s request is processed before the snoop&#39;s request. By this definition all conflicts with writeback requests can be treated as late conflicts because the writeback is processed first. Otherwise, data consistency and coherency could be violated if the home agent were to process the request before the writeback commits to memory. Because all conflicts with writebacks are deemed late conflicts, coherence agents can be configured to block conflicting snoops until an outstanding writeback request completes. Further, writebacks can also block the processing of forwards. Blocking forwards by an active writeback can also be implemented as a protocol requirement for supporting uncacheable stores, among other examples. 
     When a coherence agent receives a request to snoop its cache, it can first check if the coherence protocol will allow it, and then it may process the snoop and generate a response. One or more state tables can be defined within a set of state tables that defines the protocol specification. One or more state table can specify when a coherence agent may process a snoop and whether it will snoop the cache or instead generate a conflict forward request. In one example, there are two conditions under which a coherence agent processes a snoop. The first condition is when the coherence agent has a REQ request (Rd* or Inv*) to the snoop address and it has an available forward resource. In this case, the coherence agent must generate a forward request (RspCnflt). The second condition is when the coherence agent does not have a REQ, Wb*, or EvctCln request to the snoop address. A state table can define how a coherence agent is to process the snoop in accordance with such respective conditions. In one example, under other conditions, the coherence agent can block the snoop until either a forward resource becomes available (first condition) or the blocking Wb* or EvctCln receives its CmpU response (second condition). Note that NonSnp* requests may not affect snoop processing and a coherence agent can disregard NonSnp* entries when determining how to process or block a snoop. 
     When generating a forward request, a coherence agent can reserve a resource for the forward response. The HPI Coherence protocol, in one example, may not require a minimum number of forward response resources (beyond having at least one) and can allow a coherence agent to block snoops when it has no forward response resources available. 
     How a coherence agent processes a snoop in its cache can depend upon the snoop type and current cache state. For a given snoop type and cache state, however, there may be many allowed responses. For example, a coherence agent with a full modified line that receives a non-conflicting SnpMigr (or is processing a forward response after a SnpMigr) may do any of the following: Downgrade to S, send implicit writeback to Home and Data_F to requestor; Downgrade to S, send implicit writeback to Home; Downgrade to I, send Data_M to requestor; Downgrade to I, send implicit writeback to Home and Data_E to requestor; Downgrade to I, send implicit writeback to Home; among potentially other examples. 
     The HPI Coherence protocol allows a coherence agent to store modified lines with partial masks in its cache. However, all rows in for M copies can require a Full or Empty mask. The HPI Coherence protocol, in one example, may restrict implicit writeback of partial lines. A coherence agent wishing to evict a partial M line due to a snoop request (or forward response) can first initiate an explicit writeback and block the snoop (or forward) until the explicit writeback is completed. 
     Saving information for forward responses: The HPI Coherence Protocol, in one embodiment, allows a coherence agent to store forward response information separate from the outgoing request buffer (ORB). Separating the information allows the ORB to release ORB resources and RTID when all responses are gathered, regardless of the entry being involved in a conflict. State tables can be utilized to specify what information to store for forward responses and under what conditions. 
     Forward responses in the HPI Coherence protocol can contain the address, the requesting agent&#39;s NID, and the home TID. It does not contain the original snoop type or the RTID. A coherence agent may store the forward type and the RTID if it wishes to use them with the forward response, and it may use the address to match the incoming forward response with the proper forward entry (and to generate the home NID). Storing the forward type may be optional. If no type is stored, the coherence agent can treat a forward response as having FwdInv type. Likewise, storing the RTID can be optional and may only occur when the coherence agent is to support cache-to-cache transfers when processing forward responses. 
     As noted above, coherence agents can generate a forward request when a snoop request conflicts with an outstanding request. Forward requests target the home agent that generated the snoop, which can be determined from the address in the snoop request. Thus, the destNID can identify a home agent. The forward request can also include the TID for the home agent&#39;s resource allocated to process the original request and the NID of the coherence agent generating the forward request. 
     In one embodiment, a coherence agent can block forwards for writeback requests to maintain data consistency. Coherence agents can also use a writeback request to commit uncacheable (UC) data before processing a forward and can allow the coherence agent to writeback partial cache lines instead of protocol supporting a partial implicit writeback for forwards. Indeed, in one embodiment, a coherence agent can be allowed to store modified lines with partial masks in its cache (although M copies are to include a Full or Empty mask). 
     In one example, early conflicts may be resolved by a forward response encountering an outstanding standard request before it has received any response. A corresponding protocol state table can specify, in one example, that a forward response can be processed as long as the standard request entry is still in ReqSent state. Late conflicts can be resolved by a forward response arriving after the outstanding request has received its completion response. When this occurs either the request will have finished (already received its data or was an Inv* request) or the entry is in its RcvdCmp state. If the request is still waiting for its data, then the coherence agent must block the forward until the data is received (and used). If the conflicting Rd* or Inv* request has finished, then the forward response may be processed as long as the coherence agent has not initiated an explicit writeback of the cache line. It can be permissible for a coherence agent to initiate an explicit writeback while it has a forward response (or snoop request) to the same address, thus allowing partial lines (e.g. Snoop Requests to Partially Modified Lines”) or uncacheable stores to be properly committed to memory. 
     Turning to  FIG. 7 , a first example is illustrated of an example conflict management scheme. A first cache (or coherence) agent  705  can send a read request for a particular line of data to home agent  710  resulting in a read of memory  715 . Shortly after the read request by cache agent  705 , another cache agent  720  makes a request for ownership (RFO) of the same line. However, the home agent  710  has sent the Data_S_CmpO to the first cache agent  705  prior to receiving the RFO from cache agent  720 . The RFO can result in a snoop (SnpFO) being sent to the cache agent  705  (as well as other cache agents), the snoop being received by the first cache agent  705  prior to receiving the complete Data_S_CmpO. The cache agent  705 , upon receiving the snoop SnpO can identify a potential conflict involving the line of memory requested in its original read request and can notify the home agent  710  of the conflict by responding to the SnpO with a forward responses conflict message (RspCnflt). The home agent  710  can respond to the forward response RspCnflt by sending a forward response (FwdCnfltO). The cache agent  705  can then receive the shared data complete Data_S_CmpO and transition from an I state to S state. The forward response FwdCnfltO can then be received by the cache agent  705  and cache agent  705  can determine how to respond to the forward response message FwdClfltO based on the snoop SnpFO that triggered the sending of the forward response RspCnflt. In this example, the cache agent  705  can consult a protocol state table, for instance, to determine a response to the forward response message FwdClfltO. In the particular example of  FIG. 7 , the cache agent  705  can transition to an F-state and send the S-copy of the data it received from the home agent  710  in the Data_S_CmpO message to the second cache agent  720  in a Data_F message. The first cache agent  705  can also send a response message RspFwdS to the home agent  710  notifying the home agent  710  that the first cache agent has shared its copy of the data with the second cache agent. 
     In another illustrative example, shown in the simplified flow diagram of  FIG. 8 , the first cache agent  705  can send a request for ownership (RFO) of a particular line of memory to the home agent  710 . Shortly thereafter, a second cache agent can send a RdInvOwn message to the home agent  710  as a request for the same line of memory in an M state. In connection with the RFO message from the first cache agent  705 , the home agent  710  can send a snoop (SnpFO) to the second cache agent  720  which the second cache agent  720  can identify as a potential conflict involving the line of memory subject to both the RFO and RdInvOwn requests. Accordingly, the second cache agent  720  can send a forward request RspCnflt to the home agent  720 . The home agent  720  responds to the second cache agent&#39;s  720  forward request with a forward response. The second cache agent  720  determines a response to the forward response based on information contained in the original snoop SnpFO. In this example, the second cache agent  720  responds with a snoop response RspI indicating that the second cache agent  720  is in an I-state. The home agent  710  receives the snoop response RspI and determines that it is appropriate to send the data complete exclusive (Data_E_CmpO) to the first cache agent  705 , which causes the first cache agent to transition to an E state. With the complete sent, the home agent  710  can then begin responding to the second cache agent&#39;s RdInvOwn request, beginning with a snoop request SnpInvO of the first cache agent  705 . The first cache agent  705  can identify that the snoop results in a request by the second cache agent  720  to obtain an exclusive M-state copy of the line. Consequently, the first cache agent  705  transitions to the M state to send its copy of the line as an M-state copy (with Data_M message) to the second cache agent  720 . Additionally, the first cache agent  705  also sends a response message RspFwdI to indicate that the copy of the line has been sent to the second cache agent  720  and that the first cache agent has transitioned to an I-state (having given up ownership of the copy to the second cache agent  720 ). 
     Turning next to the example of  FIG. 9 , another simplified flowchart is shown. In this example, a cache agent  720  attempts to request exclusive ownership of an uncacheable (UC) line without receiving data (e.g., through a InvItoE message). A first cache agent  705  send a competing message (RdInv) for the cache line in E state. The HPI Coherence protocol can specify that if the requested line was previously cached in M state, the line will be written to memory before E data is delivered in response to the RdInv of the first cache agent  705 . The home agent  710  can send a complete (CmpO) to the InvItoE request and send a snoop (SnpInv) to cache agent  720  based on the RdInv request. If the cache agent  720  receives the snoop before the complete, the cache agent  720  can identify that the snoop pertains to the same cache line as its exclusive ownership request and indicate a conflict through a forward requests RspCnflt. As in previous examples, the home agent  710  can be configured to respond to the forward request with a forward response (FwdCnfltO). Multiple permissible responses may be allowed to the forward response. For instance, the cache agent  720  can initiate an explicit writeback (e.g., WbMtoI) and block the snoop (or forward) until the explicit writeback is completed (e.g., CmpU), as shown in the example of  FIG. 9 . The cache agent can then complete the snoop response (RspI). The home agent  710  can then process the RdInv request of the first cache agent  705  and return a complete Data_E_CmpO, among other examples. 
     In examples, such as the example of  FIG. 9 , where a cache agent receives a snoop when the agent has an outstanding read or invalidation request to the same address and it has cached a partial modified line (often referred to as a “buried-M”), the HPI Coherence protocol, in one implementation, allows the agent to either 1) perform an explicit writeback (partial) of the line while blocking the snoop or 2) send a forward request (RspCnflt) to the home agent. If (1) is chosen, the agent processes the snoop after receiving the complete for the writeback. If (2) is chosen, it is possible that the agent will receive forward response (FwdCnfltO) while its outstanding read or invalidation request is still waiting for responses and the agent still has a partial modified line. If that is the case, the protocol allows the agent to block the forward while performing an explicit writeback (partial) of the line. During the writeback, the protocol guarantees the agent will not receive responses for the outstanding read or invalidation request. The mechanism described above (allowing coherence agents to issue explicit writebacks and block snoops and forwards, even when the agent has an outstanding read or invalidation request) is also used to ensure partial or UC writes are posted to memory before the writer acquires global observability. 
     Coherence agents use a two-step process for partial/UC writes. First, they check if they have ownership of the cacheline and issue an ownership (invalidation) request in the protocol if they do not. Second, they perform the write. In the first step, if they performed an ownership request, it is possible that the request will conflict with other agents&#39; requests for the line, meaning the agent might receive a snoop while the ownership request is outstanding. Per coherence protocol requirements, the agent will issue a forward request for the conflicting snoop. While waiting for the forward response, the agent may receive the ownership request&#39;s completion, which grants ownership of the line to the agent and allows the agent to initiate the writeback for the partial/UC write. While this is occurring, the agent might receive the forward response, which it is obligated to process also. The coherence agent may not combine the two activities. The coherence agent is to instead writeback the partial/UC write data separately from processing the forward, and perform the writeback first. For instance, a cache agent may use a writeback request to commit UC data before processing forward and writeback partial cache lines, among other examples and features. 
     HPI can be incorporated in any variety of computing devices and systems, including mainframes, server systems, personal computers, mobile computers (such as tablets, smartphones, personal digital systems, etc.), smart appliances, gaming or entertainment consoles and set top boxes, among other examples. For instance, referring to  FIG. 10 , an embodiment of a block diagram for a computing system including a multicore processor is depicted. Processor  1000  includes any processor or processing device, such as a microprocessor, an embedded processor, a digital signal processor (DSP), a network processor, a handheld processor, an application processor, a co-processor, a system on a chip (SOC), or other device to execute code. Processor  1000 , in one embodiment, includes at least two cores—core  1001  and  1002 , which may include asymmetric cores or symmetric cores (the illustrated embodiment). However, processor  1000  may include any number of processing elements that may be symmetric or asymmetric. 
     In one embodiment, a processing element refers to hardware or logic to support a software thread. Examples of hardware processing elements include: a thread unit, a thread slot, a thread, a process unit, a context, a context unit, a logical processor, a hardware thread, a core, and/or any other element, which is capable of holding a state for a processor, such as an execution state or architectural state. In other words, a processing element, in one embodiment, refers to any hardware capable of being independently associated with code, such as a software thread, operating system, application, or other code. A physical processor (or processor socket) typically refers to an integrated circuit, which potentially includes any number of other processing elements, such as cores or hardware threads. 
     A core often refers to logic located on an integrated circuit capable of maintaining an independent architectural state, wherein each independently maintained architectural state is associated with at least some dedicated execution resources. In contrast to cores, a hardware thread typically refers to any logic located on an integrated circuit capable of maintaining an independent architectural state, wherein the independently maintained architectural states share access to execution resources. As can be seen, when certain resources are shared and others are dedicated to an architectural state, the line between the nomenclature of a hardware thread and core overlaps. Yet often, a core and a hardware thread are viewed by an operating system as individual logical processors, where the operating system is able to individually schedule operations on each logical processor. 
     Physical processor  1000 , as illustrated in  FIG. 10 , includes two cores—core  1001  and  1002 . Here, core  1001  and  1002  are considered symmetric cores, i.e. cores with the same configurations, functional units, and/or logic. In another embodiment, core  1001  includes an out-of-order processor core, while core  1002  includes an in-order processor core. However, cores  1001  and  1002  may be individually selected from any type of core, such as a native core, a software managed core, a core adapted to execute a native Instruction Set Architecture (ISA), a core adapted to execute a translated Instruction Set Architecture (ISA), a co-designed core, or other known core. In a heterogeneous core environment (i.e. asymmetric cores), some form of translation, such a binary translation, may be utilized to schedule or execute code on one or both cores. Yet to further the discussion, the functional units illustrated in core  1001  are described in further detail below, as the units in core  1002  operate in a similar manner in the depicted embodiment. 
     As depicted, core  1001  includes two hardware threads  1001   a  and  1001   b , which may also be referred to as hardware thread slots  1001   a  and  1001   b . Therefore, software entities, such as an operating system, in one embodiment potentially view processor  1000  as four separate processors, i.e., four logical processors or processing elements capable of executing four software threads concurrently. As alluded to above, a first thread is associated with architecture state registers  1001   a , a second thread is associated with architecture state registers  1001   b , a third thread may be associated with architecture state registers  1002   a , and a fourth thread may be associated with architecture state registers  1002   b . Here, each of the architecture state registers ( 1001   a,    1001   b ,  1002   a , and  1002   b ) may be referred to as processing elements, thread slots, or thread units, as described above. As illustrated, architecture state registers  1001   a  are replicated in architecture state registers  1001   b , so individual architecture states/contexts are capable of being stored for logical processor  1001   a  and logical processor  1001   b . In core  1001 , other smaller resources, such as instruction pointers and renaming logic in allocator and renamer block  1030  may also be replicated for threads  1001   a  and  1001   b . Some resources, such as re-order buffers in reorder/retirement unit  1035 , ILTB  1020 , load/store buffers, and queues may be shared through partitioning. Other resources, such as general purpose internal registers, page-table base register(s), low-level data-cache and data-TLB  1051 , execution unit(s)  1040 , and portions of out-of-order unit  1035  are potentially fully shared. 
     Processor  1000  often includes other resources, which may be fully shared, shared through partitioning, or dedicated by/to processing elements. In  FIG. 10 , an embodiment of a purely exemplary processor with illustrative logical units/resources of a processor is illustrated. Note that a processor may include, or omit, any of these functional units, as well as include any other known functional units, logic, or firmware not depicted. As illustrated, core  1001  includes a simplified, representative out-of-order (OOO) processor core. But an in-order processor may be utilized in different embodiments. The OOO core includes a branch target buffer  1020  to predict branches to be executed/taken and an instruction-translation buffer (I-TLB)  1020  to store address translation entries for instructions. 
     Core  1001  further includes decode module  1025  coupled to fetch unit  1020  to decode fetched elements. Fetch logic, in one embodiment, includes individual sequencers associated with thread slots  1001   a ,  1001   b , respectively. Usually core  1001  is associated with a first ISA, which defines/specifies instructions executable on processor  1000 . Often machine code instructions that are part of the first ISA include a portion of the instruction (referred to as an opcode), which references/specifies an instruction or operation to be performed. Decode logic  1025  includes circuitry that recognizes these instructions from their opcodes and passes the decoded instructions on in the pipeline for processing as defined by the first ISA. For example, as discussed in more detail below decoders  1025 , in one embodiment, include logic designed or adapted to recognize specific instructions, such as transactional instruction. As a result of the recognition by decoders  1025 , the architecture or core  1001  takes specific, predefined actions to perform tasks associated with the appropriate instruction. It is important to note that any of the tasks, blocks, operations, and methods described herein may be performed in response to a single or multiple instructions; some of which may be new or old instructions. Note decoders  1026 , in one embodiment, recognize the same ISA (or a subset thereof). Alternatively, in a heterogeneous core environment, decoders  1026  recognize a second ISA (either a subset of the first ISA or a distinct ISA). 
     In one example, allocator and renamer block  1030  includes an allocator to reserve resources, such as register files to store instruction processing results. However, threads  1001   a  and  1001   b  are potentially capable of out-of-order execution, where allocator and renamer block  1030  also reserves other resources, such as reorder buffers to track instruction results. Unit  1030  may also include a register renamer to rename program/instruction reference registers to other registers internal to processor  1000 . Reorder/retirement unit  1035  includes components, such as the reorder buffers mentioned above, load buffers, and store buffers, to support out-of-order execution and later in-order retirement of instructions executed out-of-order. 
     Scheduler and execution unit(s) block  1040 , in one embodiment, includes a scheduler unit to schedule instructions/operation on execution units. For example, a floating point instruction is scheduled on a port of an execution unit that has an available floating point execution unit. Register files associated with the execution units are also included to store information instruction processing results. Exemplary execution units include a floating point execution unit, an integer execution unit, a jump execution unit, a load execution unit, a store execution unit, and other known execution units. 
     Lower level data cache and data translation buffer (D-TLB)  1050  are coupled to execution unit(s)  1040 . The data cache is to store recently used/operated on elements, such as data operands, which are potentially held in memory coherency states. The D-TLB is to store recent virtual/linear to physical address translations. As a specific example, a processor may include a page table structure to break physical memory into a plurality of virtual pages. 
     Here, cores  1001  and  1002  share access to higher-level or further-out cache, such as a second level cache associated with on-chip interface  1010 . Note that higher-level or further-out refers to cache levels increasing or getting further way from the execution unit(s). In one embodiment, higher-level cache is a last-level data cache—last cache in the memory hierarchy on processor  1000 —such as a second or third level data cache. However, higher level cache is not so limited, as it may be associated with or include an instruction cache. A trace cache—a type of instruction cache—instead may be coupled after decoder  1025  to store recently decoded traces. Here, an instruction potentially refers to a macro-instruction (i.e. a general instruction recognized by the decoders), which may decode into a number of micro-instructions (micro-operations). 
     In the depicted configuration, processor  1000  also includes on-chip interface module  1010 . Historically, a memory controller, which is described in more detail below, has been included in a computing system external to processor  1000 . In this scenario, on-chip interface  101  is to communicate with devices external to processor  1000 , such as system memory  1075 , a chipset (often including a memory controller hub to connect to memory  1075  and an I/O controller hub to connect peripheral devices), a memory controller hub, a northbridge, or other integrated circuit. And in this scenario, bus  1005  may include any known interconnect, such as multi-drop bus, a point-to-point interconnect, a serial interconnect, a parallel bus, a coherent (e.g. cache coherent) bus, a layered protocol architecture, a differential bus, and a GTL bus. 
     Memory  1075  may be dedicated to processor  1000  or shared with other devices in a system. Common examples of types of memory  1075  include DRAM, SRAM, non-volatile memory (NV memory), and other known storage devices. Note that device  1080  may include a graphic accelerator, processor or card coupled to a memory controller hub, data storage coupled to an I/O controller hub, a wireless transceiver, a flash device, an audio controller, a network controller, or other known device. 
     Recently however, as more logic and devices are being integrated on a single die, such as SOC, each of these devices may be incorporated on processor  1000 . For example in one embodiment, a memory controller hub is on the same package and/or die with processor  1000 . Here, a portion of the core (an on-core portion)  1010  includes one or more controller(s) for interfacing with other devices such as memory  1075  or a graphics device  1080 . The configuration including an interconnect and controllers for interfacing with such devices is often referred to as an on-core (or un-core configuration). As an example, on-chip interface  1010  includes a ring interconnect for on-chip communication and a high-speed serial point-to-point link  1005  for off-chip communication. Yet, in the SOC environment, even more devices, such as the network interface, co-processors, memory  1075 , graphics processor  1080 , and any other known computer devices/interface may be integrated on a single die or integrated circuit to provide small form factor with high functionality and low power consumption. 
     In one embodiment, processor  1000  is capable of executing a compiler, optimization, and/or translator code  1077  to compile, translate, and/or optimize application code  1076  to support the apparatus and methods described herein or to interface therewith. A compiler often includes a program or set of programs to translate source text/code into target text/code. Usually, compilation of program/application code with a compiler is done in multiple phases and passes to transform hi-level programming language code into low-level machine or assembly language code. Yet, single pass compilers may still be utilized for simple compilation. A compiler may utilize any known compilation techniques and perform any known compiler operations, such as lexical analysis, preprocessing, parsing, semantic analysis, code generation, code transformation, and code optimization. 
     Larger compilers often include multiple phases, but most often these phases are included within two general phases: (1) a front-end, i.e. generally where syntactic processing, semantic processing, and some transformation/optimization may take place, and (2) a back-end, i.e. generally where analysis, transformations, optimizations, and code generation takes place. Some compilers refer to a middle, which illustrates the blurring of delineation between a front-end and back end of a compiler. As a result, reference to insertion, association, generation, or other operation of a compiler may take place in any of the aforementioned phases or passes, as well as any other known phases or passes of a compiler. As an illustrative example, a compiler potentially inserts operations, calls, functions, etc. in one or more phases of compilation, such as insertion of calls/operations in a front-end phase of compilation and then transformation of the calls/operations into lower-level code during a transformation phase. Note that during dynamic compilation, compiler code or dynamic optimization code may insert such operations/calls, as well as optimize the code for execution during runtime. As a specific illustrative example, binary code (already compiled code) may be dynamically optimized during runtime. Here, the program code may include the dynamic optimization code, the binary code, or a combination thereof. 
     Similar to a compiler, a translator, such as a binary translator, translates code either statically or dynamically to optimize and/or translate code. Therefore, reference to execution of code, application code, program code, or other software environment may refer to: (1) execution of a compiler program(s), optimization code optimizer, or translator either dynamically or statically, to compile program code, to maintain software structures, to perform other operations, to optimize code, or to translate code; (2) execution of main program code including operations/calls, such as application code that has been optimized/compiled; (3) execution of other program code, such as libraries, associated with the main program code to maintain software structures, to perform other software related operations, or to optimize code; or (4) a combination thereof. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention. 
     A design may go through various stages, from creation to simulation to fabrication. Data representing a design may represent the design in a number of manners. First, as is useful in simulations, the hardware may be represented using a hardware description language or another functional description language. Additionally, a circuit level model with logic and/or transistor gates may be produced at some stages of the design process. Furthermore, most designs, at some stage, reach a level of data representing the physical placement of various devices in the hardware model. In the case where conventional semiconductor fabrication techniques are used, the data representing the hardware model may be the data specifying the presence or absence of various features on different mask layers for masks used to produce the integrated circuit. In any representation of the design, the data may be stored in any form of a machine readable medium. A memory or a magnetic or optical storage such as a disc may be the machine readable medium to store information transmitted via optical or electrical wave modulated or otherwise generated to transmit such information. When an electrical carrier wave indicating or carrying the code or design is transmitted, to the extent that copying, buffering, or re-transmission of the electrical signal is performed, a new copy is made. Thus, a communication provider or a network provider may store on a tangible, machine-readable medium, at least temporarily, an article, such as information encoded into a carrier wave, embodying techniques of embodiments of the present invention. 
     A module as used herein refers to any combination of hardware, software, and/or firmware. As an example, a module includes hardware, such as a micro-controller, associated with a non-transitory medium to store code adapted to be executed by the micro-controller. Therefore, reference to a module, in one embodiment, refers to the hardware, which is specifically configured to recognize and/or execute the code to be held on a non-transitory medium. Furthermore, in another embodiment, use of a module refers to the non-transitory medium including the code, which is specifically adapted to be executed by the microcontroller to perform predetermined operations. And as can be inferred, in yet another embodiment, the term module (in this example) may refer to the combination of the microcontroller and the non-transitory medium. Often module boundaries that are illustrated as separate commonly vary and potentially overlap. For example, a first and a second module may share hardware, software, firmware, or a combination thereof, while potentially retaining some independent hardware, software, or firmware. In one embodiment, use of the term logic includes hardware, such as transistors, registers, or other hardware, such as programmable logic devices. 
     Use of the phrase ‘configured to,’ in one embodiment, refers to arranging, putting together, manufacturing, offering to sell, importing and/or designing an apparatus, hardware, logic, or element to perform a designated or determined task. In this example, an apparatus or element thereof that is not operating is still ‘configured to’ perform a designated task if it is designed, coupled, and/or interconnected to perform said designated task. As a purely illustrative example, a logic gate may provide a 0 or a 1 during operation. But a logic gate ‘configured to’ provide an enable signal to a clock does not include every potential logic gate that may provide a 1 or 0. Instead, the logic gate is one coupled in some manner that during operation the 1 or 0 output is to enable the clock. Note once again that use of the term ‘configured to’ does not require operation, but instead focus on the latent state of an apparatus, hardware, and/or element, where in the latent state the apparatus, hardware, and/or element is designed to perform a particular task when the apparatus, hardware, and/or element is operating. 
     Furthermore, use of the phrases ‘to,’ ‘capable of/to,’ and or ‘operable to,’ in one embodiment, refers to some apparatus, logic, hardware, and/or element designed in such a way to enable use of the apparatus, logic, hardware, and/or element in a specified manner. Note as above that use of to, capable to, or operable to, in one embodiment, refers to the latent state of an apparatus, logic, hardware, and/or element, where the apparatus, logic, hardware, and/or element is not operating but is designed in such a manner to enable use of an apparatus in a specified manner. 
     A value, as used herein, includes any known representation of a number, a state, a logical state, or a binary logical state. Often, the use of logic levels, logic values, or logical values is also referred to as 1&#39;s and 0&#39;s, which simply represents binary logic states. For example, a 1 refers to a high logic level and 0 refers to a low logic level. In one embodiment, a storage cell, such as a transistor or flash cell, may be capable of holding a single logical value or multiple logical values. However, other representations of values in computer systems have been used. For example the decimal number ten may also be represented as a binary value of 1010 and a hexadecimal letter A. Therefore, a value includes any representation of information capable of being held in a computer system. 
     Moreover, states may be represented by values or portions of values. As an example, a first value, such as a logical one, may represent a default or initial state, while a second value, such as a logical zero, may represent a non-default state. In addition, the terms reset and set, in one embodiment, refer to a default and an updated value or state, respectively. For example, a default value potentially includes a high logical value, i.e. reset, while an updated value potentially includes a low logical value, i.e. set. Note that any combination of values may be utilized to represent any number of states. 
     The embodiments of methods, hardware, software, firmware or code set forth above may be implemented via instructions or code stored on a machine-accessible, machine readable, computer accessible, or computer readable medium which are executable by a processing element. A non-transitory machine-accessible/readable medium includes any mechanism that provides (i.e., stores and/or transmits) information in a form readable by a machine, such as a computer or electronic system. For example, a non-transitory machine-accessible medium includes random-access memory (RAM), such as static RAM (SRAM) or dynamic RAM (DRAM); ROM; magnetic or optical storage medium; flash memory devices; electrical storage devices; optical storage devices; acoustical storage devices; other form of storage devices for holding information received from transitory (propagated) signals (e.g., carrier waves, infrared signals, digital signals); etc, which are to be distinguished from the non-transitory mediums that may receive information there from. 
     Instructions used to program logic to perform embodiments of the invention may be stored within a memory in the system, such as DRAM, cache, flash memory, or other storage. Furthermore, the instructions can be distributed via a network or by way of other computer readable media. Thus a machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer), but is not limited to, floppy diskettes, optical disks, Compact Disc, Read-Only Memory (CD-ROMs), and magneto-optical disks, Read-Only Memory (ROMs), Random Access Memory (RAM), Erasable Programmable Read-Only Memory (EPROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), magnetic or optical cards, flash memory, or a tangible, machine-readable storage used in the transmission of information over the Internet via electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). Accordingly, the computer-readable medium includes any type of tangible machine-readable medium suitable for storing or transmitting electronic instructions or information in a form readable by a machine (e.g., a computer). 
     The following examples pertain to embodiments in accordance with this Specification. One or more embodiments may provide an apparatus, a system, a machine readable storage, a machine readable medium, and a method to send a coherence protocol message corresponding to a particular cache line, identify a potential conflict involving the particular cache line, and send a forward request to a home agent to identify the potential conflict. 
     One or more examples can further provide receiving a snoop corresponding to the particular cache line. 
     One or more examples can further provide identifying that the snoop is received while a request is outstanding, and the potential conflict is identified based on identifying that the snoop is received while the request is outstanding. 
     One or more examples can further provide receiving a forward response from the home agent based on the forward request. 
     One or more examples can further provide determining a response to the forward response based at least in part on attributes of the snoop. 
     In at least one example, the snoop corresponds to another coherence protocol message by another agent corresponding to the particular cache line, and the attributes of the snoop include identification of the other agent, identification of a command included in the other coherence protocol message, and a transaction identifier of the other coherence protocol message. 
     In at least one example, the response to the forward response includes a snoop response and the protocol layer logic is further to send the snoop response to the home agent after receiving a completion to the coherence protocol message. 
     In at least one example, the response comprises performing a writeback to memory prior to sending a snoop response to the home agent. 
     In at least one example, the particular cache line is partially modified at the agent. 
     One or more examples can further provide receiving a completion after receiving the forward response, 
     One or more examples can further provide receiving a completion before receiving the forward response. 
     One or more examples can further provide allocating a resource for responses to the request. 
     One or more examples can further provide allocating a forward resource for a forward responses to the forward request. 
     One or more embodiments may provide an apparatus, a system, a machine readable storage, a machine readable medium, and a method to receive a first coherence protocol request from a first cache agent, send a snoop request to a second cache agent, wherein the snoop request corresponds to the first coherence protocol request, receive a forward request from the second cache agent corresponding to the snoop request, wherein the forward request identifies a potential conflict with the first coherence protocol request, and send a forward response to the second cache agent in response to the forward request. 
     One or more examples can further provide receiving another coherence protocol request from the second cache agent, wherein the first coherence protocol request and the other coherence protocol request each pertain to a common cache line. 
     In at least one example, the other coherence protocol request is received by the agent before the first coherence protocol request, and the protocol layer logic is further to process the other coherence protocol request and return a complete message to the other coherence protocol request. 
     One or more examples can further provide receiving a response from the second agent to the forward response and generate a complete to the first coherence protocol request upon receiving the response to the forward response. 
     In at least one example, the agent comprises a home agent. 
     One or more examples can further provide system with an interconnect fabric, a home agent to service requests to a coherent memory space, and a cache agent communicatively coupled to the home agent via the interconnect fabric. The interconnect fabric can ensure ordering of the response to the snoop and the complete for the other coherency protocol request. The home agent can have a set of resources and the set of resources are not pre-allocated to cache agents in the system. 
     One or more examples can further provide an agent including a layered protocol stack including a protocol layer, wherein the protocol layer is to initiate an allocation of resources, without home agent intervention, to process a first request in response to the agent receiving the first request, and to initiate an allocation of resources, without home agent intervention, to process responses to a second request in response to the agent transmitting the second request. 
     In at least one example, allocation of resources includes on of: HTID, RNID, RTID, or a combination thereof. 
     In at least one example, allocation of resources includes on of: resources to process snoop requests and forward responses. 
     One or more examples can further provide an agent including a layered protocol stack including a protocol layer, wherein the protocol layer is to imitate use of an ordered response channel to perform conflict resolution. 
     One or more examples can further provide a coherence agent including a layered protocol stack including a protocol layer, wherein the protocol layer is to block a forward for a writeback request to maintain data consistency. 
     In at least one example, the protocol layer is to initiate a writeback request to commit unchacheable data before processing the forward 
     In at least one example, the protocol layer is further to support explicit writeback of partial cache lines. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     In the foregoing specification, a detailed description has been given with reference to specific exemplary embodiments. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. Furthermore, the foregoing use of embodiment and other exemplarily language does not necessarily refer to the same embodiment or the same example, but may refer to different and distinct embodiments, as well as potentially the same embodiment.