Abstract:
Embodiments of apparatuses, systems, and methods for single stepping a virtual machine guest using a reorder buffer are disclosed. In one embodiment, an apparatus includes a sequencer and a reorder buffer. The sequencer is to issue micro-operations. The reorder buffer is to signal the sequencer to signal the sequencer to issue micro-operations corresponding to a monitor trap flag event.

Description:
FIELD 
     Embodiments of the invention relate to virtual machine extension (VMX) architecture. More particularly, embodiments of the invention relate to a method and apparatus to enable single stepping a guest application in a virtual machine environment. 
     BACKGROUND 
     Virtual machine extension (VMX) architecture allows multiple software programs and operating systems to use the same microprocessor logic (“hardware”) by allocating processor resources to various software applications and operating systems at different times. One feature of the VMX architecture allows software programs (“guest software”) to be executed one instruction at a time. Executing instructions of a software program one at a time is typically used as a debug technique and may be referred to as “single stepping” the program. 
     The VMX architecture typically uses a virtual machine monitor (VMM) program that interfaces one or more software programs, such as a virtual machine (VM), to a single microprocessor or collection of processing elements. The guest software running on each VM may include a guest operating system and various guest software applications. Furthermore, an application and operating system running on a VMM may be collectively called a VM or a guest.  FIG. 1  illustrates a prior art structure (“virtual machine environment”) for interfacing guest software to a microprocessor. Specifically,  FIG. 1  illustrates a VMM that interface two VMs (“guest software”) to a microprocessor. The software running within each VM may include a guest operating system as well as various software applications. In order to interface each VM to processor resources, such as registers, memory, and input/output (“I/O”) resources, state and control information is modified and otherwise tracked via a set of fields within a virtual machine control structure (VMCS). More specifically, a control structure, such as the VMCS, is typically used to pass control of and access to processor resources between the VMM and a VM guest. 
     One feature of the VMX architecture is the ability to single step through guest software. In order to single step through guest software, the VMM typically, among other things, sets a control field or bit, such as a called Monitor Trap Flag (MTF), in the VMCS. 
     After the MTF bit is set in the VMCS, the VMM may request a transfer of access and/or control of processor resources to the guest software. The VMM may request access to and/or control of processing resources by issuing an instruction, such as a VMLAUNCH or VMRESUME instruction. A VMLAUNCH instruction is typically executed in order to execute a guest application for the first time. Thereafter, a VMRESUME instruction may be used to perform the same function. 
     Prior art techniques of single stepping through a guest software routine or routines can often be susceptible failure if an event, such as an exception, occurs during the execution of the guest software. Typically, successful execution of instructions within a guest software context yields control to the VMM whenever a VM exit event occurs. However, some VMM instructions, such as VMLAUNCH and VMRESUME, which transfer control to guest, do not. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
         FIG. 1  illustrates a virtual machine environment, in which one embodiment of the invention may be used. 
         FIG. 2  illustrates a computer system in which at least one embodiment of the invention may be implemented. 
         FIG. 3  illustrates a point-to-point (PtP) computer system in which one embodiment of the invention may be implemented. 
         FIG. 4  illustrates an event handling technique within a re-order buffer (ROB) to facilitate single stepping of a guest application according to one embodiment of the invention. 
         FIG. 5  illustrates processor architecture logic to facilitate monitor trap flag (MTF) event detection and servicing according to one embodiment of the invention. 
         FIG. 6  is a flow diagram illustrating a technique to facilitate single stepping of a guest application according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention relate to virtual machine extension (VMX) architecture. More particularly, embodiments of the invention relate to a method and apparatus to enable single stepping of a guest application within a microprocessor or group of processing elements. 
     At least one embodiment of the invention described herein involves the use of event detection logic within a processor re-order buffer (ROB) to assist in single stepping guest software. Furthermore, at least one embodiment of the invention involves the use of prioritization logic within the ROB to assign appropriate priority to various events while single-stepping a guest software application. 
       FIG. 2  illustrates a computer system in which at least one embodiment of the invention may be used. A processor  205  accesses data from a level one (L1) cache memory  210  and main memory  215 . In other embodiments of the invention, the cache memory may be a level two (L2) cache or other memory within a computer system memory hierarchy. Illustrated within the processor of  FIG. 2  is one embodiment of the invention  206 . Other embodiments of the invention, however, may be implemented within other devices within the system, such as a separate bus agent, or distributed throughout the system in hardware, software, or some combination thereof. 
     The main memory may be implemented in various memory sources, such as dynamic random-access memory (DRAM), a hard disk drive (HDD)  220 , or a memory source located remotely from the computer system via network interface  230  containing various storage devices and technologies. The cache memory may be located either within the processor or in close proximity to the processor, such as on the processor&#39;s local bus  207 . Furthermore, the cache memory may contain relatively fast memory cells, such as a six-transistor (6T) cell, or other memory cell of approximately equal or faster access speed. 
     The computer system of  FIG. 2  may be a point-to-point (PtP) network of bus agents, such as microprocessors, that communicate via bus signals dedicated to each agent on the PtP network. Within, or at least associated with, each bus agent is at least one embodiment of invention  206 , such that store operations can be facilitated in an expeditious manner between the bus agents. 
       FIG. 3  illustrates a computer system that is arranged in a point-to-point (PtP) configuration. In particular,  FIG. 3  shows a system where processors, memory, and input/output devices are interconnected by a number of point-to-point interfaces. 
     The  FIG. 3  system may also include several processors, of which only two, processors  370 ,  380  are shown for clarity. Processors  370 ,  380  may each include a local memory controller hub (MCH)  372 ,  382  to connect with memory  32 ,  34 . Processors  370 ,  380  may exchange data via a point-to-point interface  350  using point-to-point interface circuits  378 ,  388 . Processors  370 ,  380  may each exchange data with a chipset  390  via individual point-to-point interfaces  352 ,  354  using point to point interface circuits  376 ,  394 ,  386 ,  398 . Chipset  390  may also exchange data with a high-performance graphics circuit  338  via a high-performance graphics interface  392 . 
     At least one embodiment of the invention may be located within the memory controller hub  372  or  382  of the processors. Other embodiments of the invention, however, may exist in other circuits, logic units, or devices within the system of  FIG. 3 . Furthermore, other embodiments of the invention may be distributed throughout several circuits, logic units, or devices illustrated in  FIG. 3 . 
       FIG. 4  illustrates an event handling technique within a re-order buffer (ROB) to facilitate single stepping of a guest application according to one embodiment of the invention. Internal events  401  or external events  405  are detected  410  by detection logic within the ROB. After the events are detected, they are prioritized  415  so as to allow for the proper handling of the events. For example, in one embodiment of the invention, an MTF event initiated by a guest instruction will be prioritized by prioritization logic within the ROB above other internal or external events so that VMM may use the processor resource before servicing those events. The ROB allows a sequencer to issue the appropriate micro-operations (uops) to service the event(s). 
       FIG. 5  illustrates a microprocessor architecture for implementing at least one embodiment of the invention. The processor architecture  500  of  FIG. 5  has a sequencer  501  that issues uops associated with a particular instruction to an execution unit  505 . The instructions are retrieved from memory via an instruction fetch unit  510 , which provides these instructions to the sequencer in order to translate the instruction to uops. Instructions and their associated uops may be executed out of order, but the ROB  515  retires the uops and instructions in program order. 
     In one embodiment of the invention, the ROB detects and prioritizes the uops retrieved from the execution unit and signals the sequencer to issue instructions commensurate with the detection and prioritization. In order for the ROB to detect an event, such as an MTF event caused by a guest instruction, two control bits are used in one embodiment. The control bits, MTF — 1 and MTF — 2, are set by an instruction associated with the MTF event to indicate that a guest instruction wishes to gain use of processor resources. 
     In at least one embodiment, after setting the MTF control bit in the execution control information in VMCS, VMM requests the transfer of control of processor resources to guest software by issuing a VMLAUNCH or VMRESUME instruction. The VMLAUNCH or VMRESUME instruction (whichever is issued) is then decoded into individual micro-operations (uops). 
     During execution of the first uop of VMLAUNCH or VMRESUME, the MTF — 1 and MTF — 2 bits are set to the same value, such as a logical “0” value. Subsequent uops may set the MTF — 1 to another value, such as a logical “1” value. If the last uop of VMLAUNCH or VMRESUME, which is indicated by an encoded label, ENDMACRO — 1, is retired having MTF — 1=1 and MTF — 2=0, for example, then no MTF event is generated and the processor may start executing first guest instruction. 
     The ROB recognizes the beginning of the uops associated with the first guest instruction by the presence of a label associated with the first uop of the guest instruction, such as “BEGINMACRO”. After the ROB has detected the encoded label, it sets MTF — 1 and MTF — 2 bits to the same value, such as a logical “1” value. 
     The ROB can recognize the end of the guest instruction by detecting another label associated with the last uop of the first guest instruction, such as “ENDMACRO_n”. After detecting either set MTF control bit and the ENDMACRO_n label, the ROB determines which event among all concurrent events being detected is highest priority and allows the sequencer to issue uops commensurate with the appropriately prioritized event. The logic performed by the ROB to perform the detection and prioritization is illustrated by the pseudo-code  517  in  FIG. 5 . 
     For example, if the MTF event caused by the detection of the last uop associated with the guest instruction is highest priority among all concurrently detected events, the ROB allows the sequencer to issue uops associated with an MTF event handling routine. Otherwise, the ROB may signal the sequencer to issue uops associated with a higher priority event, such as a fault or exception. 
     In order to transition from the execution of a guest instruction, such as in the case of single-stepping, the ROB, in at least one embodiment, will signal the sequencer to issue uops associated with a virtual machine (VM) exit routine, which, among other things, will reset the MTF control bits inside the ROB to “0”, for example, in order to prevent another MTF event before another VMLAUNCH or VMRESUME instruction is issued by VMM. 
       FIG. 6  is a flow diagram illustrating a method for performing at least one embodiment of the invention. At operation  601 , the VMAUNCH or VMRESUME instruction is fetched and the MTF — 1 bit is cleared to a “0” state, and VMLAUNCN/VMRESUME is decoded into a set of micro-operations (uops). The VMLAUNCH or VMRESUME instruction is then executed and the MTF — 2 and MTF — 1 bits are set to a “0” value at operation  605 . Subsequent uops decoded from the VMRESUME/MLAUNCH instruction may be issued and executed, in which case the MTF — 1 bit may be set to a “1” state and no MTF event will be detected. When subsequent uops decoded from VMLAUNCH/VMRESUME do in fact issue and are executed, MTF — 1 will be set to a “1” value and MTF — 2 will be set to a “0” value. Consequently, when the last uop decoded from VMLAUNCH/VMRESUME is executed, no MTF event will be detected at operation  610 . 
     Next, the guest instruction is fetched and executed at operation  615 , and the MTF bits are set to an equal value (e.g., “1”). If the last uop of the guest instruction is executed with MTF bits equal a “1” value (indicated by an encoded label, such as ENDMACRO_n), an MTF event will be detected at operation  620 . 
     If a higher priority event occurs, a sequencer may issue operations from the higher priority event handler routine at operation  625  and the last operation of that routine will trigger an MTF event at operation  627 . If no higher priority event occurs or after the higher priority event has been handled, the sequencer may then issue operations from the MTF event handler routine at operation  630 . The MTF bits are then reset to a “0” state and the correct exit reason is saved at operation  635 . At this point, control is handed back to the VMM, which can begin fetching instructions from other program clients at operation  640 . 
     Any or all portions of the embodiments of the invention illustrated herein may be implemented in a number of ways, including, but not limited to, logic using complimentary metal-oxide-semiconductor (CMOS) circuit devices (hardware), instructions stored within a storage medium (software), which when executed by a machine, such as a microprocessor, cause the microprocessor to perform operations described herein, or a combination of hardware and software. References to “microprocessor” or “processor” made herein are intended to refer to any machine or device that is capable of performing operations as a result of receiving one or more input signals or instructions, including CMOS devices. 
     Although the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention.