Patent Publication Number: US-9892069-B2

Title: Posting interrupts to virtual processors

Description:
CLAIM OF PRIORITY 
     This application claims the priority filing benefit of, is a continuation of, and incorporates by reference, U.S. patent application Ser. No. 14/467,604 entitled “POSTING INTERRUPTS TO VIRTUAL PROCESSORS” filed on Aug. 25, 2014, which is a continuation application of U.S. patent application Ser. No. 13/837,730, entitled “POSTING INTERRUPTS TO VIRTUAL PROCESSORS” filed Mar. 15, 2013 which is a continuation of U.S. patent application Ser. No. 12/650,581, entitled “POSTING INTERRUPTS TO VIRTUAL PROCESSORS” filed Dec. 31, 2009 now patented as U.S. Pat. No. 8,566,492 issued on Oct. 22, 2013. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure pertains to the field of information processing, and more particularly, to the field of managing interrupts in an information processing system. 
     2. Description of Related Art 
     Generally, the concept of virtualization in information processing systems allows multiple instances of one or more operating systems (each, an “OS”) to run on a single information processing system, even though each OS is designed to have complete, direct control over the system and its resources. Virtualization is typically implemented by using software (e.g., a virtual machine monitor, or a “VMM”) to present to each OS a “virtual machine” (“VM”) having virtual resources, including one or more virtual processors, that the OS may completely and directly control, while the VMM maintains a system environment for implementing virtualization policies such as sharing and/or allocating the physical resources among the VMs (the “virtualization environment”). Each OS, and any other software, that runs on a VM is referred to as a “guest” or as “guest software,” while a “host” or “host software” is software, such as a VMM, that runs outside of the virtualization environment. 
     A physical processor in an information processing system may support virtualization, for example, by supporting an instruction to enter a virtualization environment to run a guest on a virtual processor (i.e., a physical processor under constraints imposed by a VMM) in a VM. In the virtualization environment, certain events, operations, and situations, such as external interrupts or attempts to access privileged registers or resources, may be intercepted, i.e., cause the processor to exit the virtualization environment so that a VMM may operate, for example, to implement virtualization policies (a “VM exit”). 
     Therefore, external interrupts may be intercepted by the VMM and routed to the appropriate virtual processor. Alternatively, a virtualization environment may provide for external interrupts to be routed to a virtual processor without a VM exit, for example, if the interrupt request is generated by an input/output (“I/O”) device assigned to the currently active VM, or if the interrupt request is an inter-processor interrupt between two virtual processors in the same VM. Whether the interrupt request causes a VM exit or not, routing the interrupt to the appropriate virtual processor may include mapping interrupts requests from a guest&#39;s view of the system to a host&#39;s view of the system. In existing information processing systems, the VMM may be responsible for remapping interrupt requests whenever a virtual processor is migrated from one physical processor to another physical processor. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention is illustrated by way of example and not limitation in the accompanying figures. 
         FIG. 1  illustrates an embodiment of the present invention in a system for posting interrupts to virtual processors. 
         FIG. 2  illustrates an embodiment of the present invention in an apparatus for posting interrupts to virtual processors. 
         FIG. 3  illustrates a message signaled interrupt format compatible with an embodiment of the present invention. 
         FIG. 4  illustrates a message signaled interrupt register format compatible with an embodiment of the present invention 
         FIG. 5  illustrates an interrupt controller redirection table register format compatible with an embodiment of the present invention. 
         FIG. 6  illustrates an interrupt remapping table entry according to an embodiment of the present invention. 
         FIG. 7  illustrates a posted-interrupt descriptor according to an embodiment of the present invention. 
         FIG. 8  illustrates an embodiment of the present invention in a method for posting interrupts to virtual processors. 
         FIG. 9  illustrates an embodiment of the present invention in a method for managing posted interrupts. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention may be embodied in systems, apparatuses, and methods for posting interrupts to virtual processors, as described below. In the description, numerous specific details, such as component and system configurations, may be set forth in order to provide a more thorough understanding of the present invention. It will be appreciated, however, by one skilled in the art, that the invention may be practiced without such specific details. Additionally, some well known structures, circuits, and the like have not been shown in detail, to avoid unnecessarily obscuring the present invention. 
     Embodiments of the present invention provide for posting, in a data structure in a memory, interrupt requests to virtual processors. Posting interrupt requests according to embodiments of the present invention may be desirable for several reasons. 
     First, as the number of physical processors and cores per system increases, the use of techniques for optimizing system performance and efficiency by migrating virtual processors among the physical processors and cores increases. Embodiments of the present invention may provide for these techniques to be less complex, by providing for all interrupt requests destined for a virtual processor to be migrated atomically. 
     Second, posting of interrupts according to embodiments of the present invention may provide for the delivery of external interrupts to a virtual processor to depend on the state of the virtual processor, potentially increasing system performance by reducing the number of VM exits. For example, an interrupt request for a virtual processor that is waiting for one of its quanta of time on a physical processor may be held pending in memory, without causing a VM exit, until the virtual processor is running. 
     Third, I/O device virtualization, for example as provided for the Peripheral Component Interconnect (“PCI”) Express Single-Root I/O Virtualization Specification (available from http://www.pcisig.org), may provide for a single physical I/O device to be configured as multiple virtual I/O devices, each assignable to a different VM. Therefore, many more interrupt sources are possible in a virtualization environment than in a conventional system. To accommodate this possibility, embodiments of the present invention may provide for many more interrupt vectors than would be available in a prior art system, by supporting a virtual vector space for each virtual processor. 
     Elements of embodiments of the invention may be implemented in hardware, software, firmware, or any combination of hardware, software, or firmware. The term hardware generally refers to an element having a physical structure such as electronic, electromagnetic, optical, electro-optical, mechanical, electro-mechanical parts, etc. The term software generally refers to a logical structure, a method, a procedure, a program, a routine, a process, an algorithm, a formula, an expression, etc. The term firmware generally refers to a logical structure, a method, a procedure, a program, a routine, a process, an algorithm, a formula, or an expression that is implemented or embodied in a hardware structure (e.g., flash memory or read only memory). Examples of firmware are microcode, writable control store, and micro-programmed structure. 
       FIG. 1  illustrates an embodiment of the present invention in information processing system  100 . Information processing system  100  includes bare platform hardware  110 , which may be any apparatus capable of executing any OS, VMM, or other software. For example, bare platform hardware  110  may be the hardware of a personal computer, a mainframe computer, a portable computer, a handheld device, a set-top box, a server, or any other computing system. In this embodiment, bare platform hardware  110  includes processors  120  and  122 , chipset  130 , system memory  140 , and device  150 . 
     Processors  120  and  122  may be any components having one or more execution cores, where each execution core may be based on any of a variety of different types of processors, including a general purpose microprocessor, such as a processor in the Intel® Pentium® Processor Family, Itanium® Processor Family, or other processor family from Intel® Corporation, or another processor from another company, or a digital signal processor or microcontroller. Although  FIG. 1  shows two processors, bare processing hardware  110  may include any number of processors, including any number of multicore processors, each with any number of execution cores, and any number of multithreaded processors, each with any number of threads. 
     Chipset  130  may be any group of circuits and logic that supports memory operations, input/output operations, configuration, control, internal or external interface, connection, or communications functions (e.g., “glue” logic and bus bridges), and/or any similar functions for processors  120  and  122  and/or system  100 . Individual elements of chipset  130  may be grouped together on a single chip, a pair of chips, dispersed among multiple chips, and/or be integrated partially, totally, redundantly, or according to a distributed approach into one or more processors, including processors  120  and/or  122 . In this embodiment, chipset  130  includes interrupt remapping unit  132  for remapping or posting interrupts according to an embodiment of the invention, as described below. In other embodiments, interrupt remapping unit  132  may be included elsewhere in system  100 . 
     System memory  140  may be any medium on which information, such as data and/or program code, may be stored, such as static or dynamic random access memory, semiconductor-based read-only or flash memory, magnetic or optical disk memory, or any other type of medium readable by processors  120  and  122 , or any combination of such mediums. 
     Device  150  may represent any type of I/O, peripheral, or other device that may be the source of an interrupt request, such as a keyboard, mouse, trackball, pointing device, monitor, printer, media card, network interface, information storage device, etc. Device  150  may be embodied in a discrete component, or may be included in an integrated component with any other devices. In one embodiment, device  150  may represent a function in a multifunctional I/O, peripheral, or other device. 
     Processors  120  and  122 , chipset  130 , system memory  140 , and device  150  may be coupled to or communicate with each other according to any known approach, such as directly or indirectly through one or more parallel, sequential, pipelined, asynchronous, synchronous, wired, wireless, or other bus or point-to-point connection or means of communication. For example, in this embodiment chipset  130  includes interface  131  to receive signals, messages, and/or transactions, such as interrupt requests, from device  150 , or transmit signals, messages, and/or transactions to device  150  and/or any other agents or components in system  100 , through any such connection or other means of communication. Similarly, device  150  includes interface  151  to transmit and/or receive signals, messages, and/or transactions to chipset  130 , and/or any other agents or components in system  100 . System  100  may also include any number of additional agents, components, or connections. 
       FIG. 2  illustrates chipset  130 , including remapping unit  132 , according to one embodiment of the present invention. Remapping unit  132  includes look-up logic  220 , routing logic  230 , and posting logic  240 . Chipset  130  also includes interface  131 , described above, and interrupt controller  210 . 
     Chipset  130  may receive an interrupt request through interface  131 . An interrupt request may also be generated from within chipset  130 , for example where a timer or other device that may generate an interrupt is included in chip set  130 . In one embodiment, an interrupt request may be received as a signal, such as a level or edge triggered interrupt signal, according to any known signaling protocol. In another embodiment, an interrupt request may be received as a message, such as a bus message or a point-to-point transaction, according to any known message, transaction, or other communication protocol. Other embodiments are possible, including an embodiment using both signal and message based interrupt requests. In such an embodiment, chipset  130  may receive both types of requests; signal based requests through input terminals and message based requests through write transactions to an address or port corresponding to a register or other storage location assigned to the interrupt controller. 
     Look-up logic  220  is to look up an entry associated with an interrupt request, e.g., from device  150 , in a data structure. Look-up logic  220  may be implemented with any logical structure or circuitry that performs a function of looking up or finding an entry in a data structure. The entry may be found using a “handle” as an entry number, address, index, pointer, or other locator of or to a particular entry in the data structure, where the handle is a value supplied directly or indirectly by the interrupt request. 
     For example, according to a message signaled interrupt (“MSI”) protocol of a PCI-Express bus, an interrupt message may include a 32-bit address field and a 32-bit data field, where bits 31:20 of the address field are set to the hexadecimal value “FEE” to indicate that the message is an interrupt request. The remaining bits of the fields may be used to indicate other information, such as the interrupt vector and the desired destination for the interrupt request. An embodiment of the present invention compatible with this protocol may use the formats shown in  FIG. 3 . 
     In  FIG. 3 , 32-bit address field  310  includes bit-fields  311 ,  312 , and  313 , and 32-bit data field  320  includes bit-field  321 . Bit-field  311  may include bits 31:20 of address field  310 , and may be set to the hexadecimal value “FEE” to indicate that the message is an interrupt request. Bit-field  312  may include bits 19:5 and bit  2  of address field  310 , and may be used to indicate a 16-bit handle value. Bit-field  313  may include bit  3  of address field  310 , and may be used to indicate a 1-bit sub-handle valid (“SHV”) value. Bit-field  321  may include bits 15:0 of data field  320 , and may be used to indicate a 16-bit sub-handle value. The use of the SHV and sub-handle values will be described below. The remaining bits of address field  310  and data field  320  may be treated as reserved or ignored. 
     In order to generate an MSI transaction in such a format, device  150 , or any other device in system  100 , including a device integrated into chip set  130 , may include a register or other storage location such as MSI register  152 , as shown in  FIG. 4 . MSI register  152  may include 32-bit address field  410  and 32-bit data field  420 . Address field  410  includes bit-fields  411 ,  412 , and  413 , and data field  420  includes bit-field  421 . Bit-field  411  may include bits 31:20 of address field  410 , and may be set to the hexadecimal value “FEE” to indicate that the message is an interrupt request. Bit-field  412  may include bits 19:5 and bit  2  of address field  410 , and may be used to indicate a 16-bit handle value. Bit-field  413  may include bit  3  of address field  410 , and may be used to indicate a 1-bit sub-handle valid (“SHV”) value. Bit-field  421  may include bits 15:0 of data field  420 , and may be used to indicate a 16-bit sub-handle value. The use of the SHV and sub-handle values will be described below. The remaining bits of address field  410  and data field  420  may be treated as reserved or ignored. 
     In an embodiment where an interrupt request is sent as a signal, the signal may be received by or passed to interrupt controller  210 . For example, interrupt controller  210  may have any number of input terminals (e.g., 24), each of which may be connected to a device (e.g., through an internal connector to a device within chipset  130 , or through an internal connecter to a pin or other terminal of chipset  130  to an external connector to a device external to chipset  130 ) that may generate an interrupt request, and interrupt controller  210  may include or have access to the same number (e.g., 24) of storage locations that may be programmed with or otherwise contain the information associated with each interrupt request, including a handle value. 
     For example, in an embodiment using where interrupt controller  210  is an I/O Advanced Programmable Interrupt Controller (“TO APIC”) according to the architecture of the Intel® Pentium® Processor Family, the redirection table (“RT”) register of the IO APIC may be programmed as shown in  FIG. 5  in order to generate interrupts that are compatible with an embodiment of the invention. In  FIG. 5 , RT entry  500  includes bit-fields  511 ,  512 ,  513 ,  514 ,  515 ,  516 ,  517 ,  518 , and  519 . Bit-field  511  includes bits 63:48 of RT entry  500 , to indicate a 16-bit handle value. Bit-fields  512 ,  513 ,  514 ,  515 ,  516 , and  517  include bits  16 ,  15 ,  14 ,  13 ,  12 , and  11 , respectively, of RT entry  500 , to indicate mask, trigger mode, remote interrupt request register, interrupt input pin polarity, delivery status, and destination mode values, respectively, and provide the functionality of the known programming model. Bit-field  518  includes bits 10:8 of RT entry  500 , to be set to logical ‘000’ or ‘111’ to indicate that the delivery mode is fixed or external, respectively. Bit-field  519  includes bits 7:0 of RT entry  500 , to indicate an 8-bit interrupt vector. 
     Returning to look-up logic  220  of  FIG. 2 , in one embodiment look-up logic  220  may use a 16-bit handle from an MSI transaction or a 16-bit handle from an IO APIC to find an entry in a single level interrupt remapping table (“IRT”) having 64K entries, each entry having 128 bits (each an “IRTE”). In another embodiment, where an MSI transaction includes an SHV that is set to a logical ‘1’, or otherwise indicates that it includes a valid sub-handle, a 16-bit sub-handle from the transaction may be applied (e.g., logically or arithmetically combined with, as a mask or offset value) to a 16-bit handle from the transaction to form an effective handle, and the effective handle may be used by look-up logic  220  to find an IRTE. The value of the effective handle may be checked to ensure that it is directing look-up logic  220  to a location within the IRT. Other embodiments may use different sizes of handles, sub-handles, effective handles, and IRTs. 
     An IRT, or any other data structure to which look-up logic  220  refers, may be stored in system memory  140 , or in any other storage area in system  100 . In some embodiments, IRTEs may be cached in a storage area in remapping unit  132  or in any other area that is temporally or physically nearer to look-up logic  220  than the IRT. The base address of the IRT may be stored in IRT address register  222 , or any other storage location accessible to look-up logic  220 . 
     In one embodiment, each IRTE may have the format shown in  FIG. 6 . In  FIG. 6 , IRTE  600  includes bit-fields  610 ,  612 ,  614 , and  616 . Bit-field  610  may include bit  15  (“PST”) to indicate whether the interrupt request directed to this IRTE is to be remapped or posted. For example, if this bit is clear (PST is ‘0’), then an interrupt request directed to this IRTE will be forwarded by routing logic  240  to a destination corresponding to a destination value in the IRTE, depending on the values of other bit-fields in the IRTE. For example, if a requestor identifier from the interrupt request does not match a source value in the IRTE, the interrupt request may be blocked. Routing logic  240  may also block the interrupt request based on other criteria. 
     However, in this embodiment, if this bit is set (PST is ‘1’), then an interrupt request directed to this IRTE will be posted by posting logic  250 , as described below. If the IRTE is designated for posting (e.g., PST is ‘1’), then bit-fields  612 ,  614 , and  616  are defined and used as set forth below, and the remaining bits of the IRTE may be treated as reserved or ignored. However, if the IRTE is designated for remapping (e.g., PST is ‘0’), then the entire IRTE is defined and used as according to the specified remapping protocol, which is not described here. 
     If the IRTE is designated for posting (e.g., PST is ‘1’), then bit-field  612 ,  614 , and  616  may be defined and used as follows. Bit-field  612  may include bit  12  (“URG”) to indicate is the interrupt request is urgent (i.e., time sensitive). The URG bit may be used to differentiate between interrupt requests that are urgent, such as from media devices, and interrupt requests that are more tolerant to interrupt processing latencies. Bit-field  614  (“Virtual Vector”) may include bits  23 : 16  to indicate an eight bit virtual interrupt vector that is assigned to this interrupt request by the guest software running on the virtual processor that is the target of this interrupt request. Bit field  616  (“Posted-Interrupt Descriptor Address”) may include bits  127 : 96  and  63 : 38  to indicate an address of a data structure for posting interrupts (a “posted-interrupt descriptor”). 
     The posted-interrupt descriptor, or any other data structure to which the IRTE for a posted interrupt request refers, may be stored in system memory  140 , or in any other storage area in system  100 . In one embodiment, a posted-interrupt descriptor may be the size of a line of a cache accessible to processors  120  and  122 , and the address may be aligned to a cache line. For example, a posted-interrupt descriptor may be 64 bytes, with an address having all zeroes in the six least significant bits, therefore bit-field  616  may be used to store bits 63:6 of an address in the address space of processors  120  and  122 . 
     Software, such as a VMM, may allocate a posted-interrupt descriptor for each virtual processor that may be the target of external interrupt requests. A posted-interrupt descriptor according to one embodiment of the present invention is illustrated in  FIG. 7 . Posted-interrupt descriptor  700  in  FIG. 7  includes bit-fields  710 ,  720 ,  730 , and  740 . 
     Bit-field  710  may include the lowest 32 bytes of the 64-byte posted-interrupt descriptor to form a 256-bit posted interrupt request register (“pIRR”). Each bit of the pIRR may correspond to one of 256 virtual interrupt vectors for the virtual processor corresponding to the posted-interrupt descriptor. Each bit of the pIRR may be set to post an interrupt request for the corresponding virtual interrupt vector. 
     Bit-field  720  may include three smaller bit-fields to indicate attributes of a notify event to be used to inform a VMM of pending posted interrupts. In one embodiment, the event used to notify a VMM that posted interrupts are pending may be a physical interrupt request to a physical processor. Therefore, using a physical processor that may support over one hundred physical interrupts, embodiments of the present invention may provide for over one hundred virtual processors per physical processor. 
     Notify event attributes bit-field  720  may include bit-fields  722 ,  724 , and  726 . Bit-field  722  (“Dest-ID”) may include 32 bits to identify the destination of the interrupt request, which, for example, may be the local APIC for the physical processor on which the virtual processor that is the target of the interrupt request is running. The physical processor to which the target virtual processor has temporal affinity may change as virtual processors are migrated, so this field may be reprogrammed, by the VMM, with a new local APIC identifier in connection with a migration for load balancing or any other reason. The physical processor to which a target virtual processor has temporal affinity at any given time may be called the “notify-CPU” in this description, as it will be the physical processor to which a notify event will be sent when there are pending posted interrupts for that virtual processor. 
     Bit-field  724  (“DM”) may include a single bit to indicate the mode for the notify event. For example, in an embodiment where the notify event is a physical interrupt, DM may indicate whether the value of Dest-ID should be interpreted as a physical or a logical identifier (“ID”). Bit-field  726  (“Physical Vector”) may include eight bits to indicate the physical vector to be used for the notify event. When a notify event is delivered to the VMM, the VMM may use the physical vector to determine which virtual processor has pending posted interrupts in its posted-interrupt descriptor. Therefore, embodiments of the present invention provide for a single physical vector per virtual processor, instead of a physical vector for each virtual interrupt. 
     Bit-field  730  (“Supress” or “S”) may include one bit to store a suppress flag to indicate whether notify events are to be suppressed when posting interrupts to this posted-interrupt descriptor. Software, such as a VMM, may set this bit at any time to suppress notify events, such as when the corresponding virtual processor is not running because it is in the scheduler wait queue, waiting for one of its quanta of time to run. Bit-field  740  (“Pending” or “P”) may include one bit to store a pending flag to indicate whether there is a pending notify event for this posted-interrupt descriptor that has not been serviced yet. If this flag is already set at the time an interrupt request is posted, then there is no need to send another notify event. This flag may be set by hardware when it sends a notify event, and cleared by software as part of servicing the notify event. 
       FIGS. 8 and 9  illustrate embodiments of the present invention in methods, specifically methods  800  for posting interrupts to virtual processors and method  900  for managing posted interrupts. Although method embodiments are not limited in this respect, reference may be made to the elements in the system embodiment of  FIG. 1  and the apparatus embodiment of  FIG. 2  to describe the method embodiments of  FIGS. 8 and 9 . 
     In box  802  of  FIG. 8 , method  800  begins. 
     In box  810  of  FIG. 8 , software such as a VMM begins to configure system  100  to support a virtualization environment, including posting interrupts to virtual processors. In box  812 , the VMM allocates a memory region for an IRT, for example by programming IRT address register  222  with the base address of a memory region for an IRT. In box  814 , the VMM begins to set up the IRT by programming each IRTE, including setting the PST bit for each IRTE for which posting (e.g., instead of remapping) will be performed. In box  816 , the VMM allocates a posted-interrupt descriptor for each virtual processor, for example by programming each IRTE for which posting will be performed with the address of a memory region for a posted-interrupt descriptor. In box  818 , the VMM programs each posted-interrupt descriptor with the desired attributes and other information. In box  820 , configuration of system  100  to support the virtualization environment, including programming of the IRT and posted-interrupt descriptors, ends. 
     In box  830 , an interrupt request, including the handle, is sent to chipset  130 . In box  832 , look-up logic  220  uses the handle from the interrupt request to find an IRTE. In box  834 , the value of the PST bit in the IRTE is determined. If the PST bit is clear, method  800  continues to box  836 , where the interrupt is remapped and forwarded, by routing logic  230 , to the destination specified in the IRTE. However, if the PST bit is set, method  700  continues to box  840 . 
     In box  840 , the Posted-Interrupt Descriptor Address from the IRTE is used, for example by posting logic  240 , to find the Posted-Interrupt Descriptor for this IRTE. 
     In box  850 , a read-modify-write operation, for example by posting logic  240 , is begun on the Posted-Interrupt Descriptor. An atomic read-modify-write operation is used to allow access to the Posted-Interrupt Descriptor by multiple sources, such as multiple instances of posting logic  240 , other interrupt posting hardware, and software such as a VMM. In box  852 , the cache line storing the Posted-Interrupt Descriptor is locked. In box  854 , the pIRR bit corresponding to the Virtual Vector from the IRTE is set (or, if already set, it remains set). 
     In box  856 , it is determined whether to generate a notify event. In one embodiment, the determination is based on the values of the URG bit in the IRTE and the Pending and Suppress bits in the Posted-Interrupt Descriptor, as follows. If the URG bit is set (i.e., the interrupt request is urgent) or the Suppress flag is not set (i.e., the VMM has not temporarily suppressed interrupt requests to this virtual processor), and the Pending bit is not set (i.e., there is not already a notify event pending for this virtual processor), then method  800  continues to box  860 , where a notify event will be sent. Otherwise, method  800  continues to box  858 , where no notify event is sent. 
     In box  860 , the Pending flag in this posted-interrupt descriptor is set. In box  862 , a notify event is sent to the destination specified by the Dest-ID and DM fields of the posted-interrupt descriptor, using the value in the Physical Vector field. For example, an interrupt message may be sent to the local APIC specified by the Dest-ID and DM fields, the interrupt message including the vector from the Physical Vector field. In box  864 , the cache line is unlocked. In box  866 , the atomic read-modify-write operation, begun in box  850 , ends. 
     In box  898 , method  800  ends. 
     Turning to  FIG. 9 , method  900  is a method for managing posted interrupts according to an embodiment of the present invention. In box  902 , method  900  begins. 
     In box  904 , software such as a VMM begins to configure system  100  to support a virtualization environment, including posting interrupts to virtual processors. In box  906 , the VMM allocates a memory region for an IRT, for example by programming IRT address register  222  with the base address of a memory region for an IRT. In box  908 , the VMM begins to set up the IRT by programming each IRTE, including setting the PST bit for each IRTE for which posting (e.g., instead of remapping) will be performed. In box  910 , the VMM allocates a posted-interrupt descriptor for each virtual processor, for example by programming each IRTE for which posting will be performed with the address of a memory region for a posted-interrupt descriptor. 
     In box  912 , the VMM programs each posted-interrupt descriptor with the desired attributes and other information. For example, the Dest-ID field of a posted-interrupt descriptor for a virtual processor set up to run on processor  120  may be programmed with the ID of the local APIC for processor  120 , the DM may be set to physical, and the Physical Vector field may be programmed with the vector to be used by processor  120  to service the physical interrupt used as the posted-interrupt notify event. In box  914 , configuration of system  100  to support the virtualization environment, including programming of the IRT and posted-interrupt descriptors, ends. 
     In box  920 , a virtual processor may begin to run on a physical processor, such as processor  120 , in system  100 . Therefore, the virtual processor is in an active state and its notify-CPU is processor  120 . In box  922 , a notify event for the virtual processor, for example a physical interrupt from box  862  of method  800 , may be received by processor  120 , causing a VM exit in box  924 . In box  926 , the VMM begin to service the physical interrupt, using the physical vector to identify the virtual processor for which the notify event was sent. In box  928 , the VMM may read the posted-interrupt descriptor for the virtual processor to determine the virtual interrupt request that is pending, according to the pIRR. In box  930 , the VMM may inject the virtual interrupt request into the virtual processor for servicing. In box  932 , the VMM may clear the Pending flag in the posted-interrupt descriptor. In box  934 , the virtual processor may be restarted on physical processor  120 . 
     In box  936 , the virtual processor&#39;s quantum may expire, so in box  938 , the virtual processor may move into a runnable state. In box  940 , the VMM may set the Suppress flag in the posted-interrupt descriptor, in order to prevent a virtual interrupt for the virtual processor from causing a VM exit while the virtual processor is in the runnable state. In box  942 , the virtual processor may be waiting for the scheduler to activate it on its notify-CPU. In box  944 , the next quantum for the virtual processor may arrive. Therefore, in box  946 , the VMM may clear the Suppress flag that it set in box  940 . 
     In box  948 , the virtual processor may begin to run on processor  120 . Therefore, the virtual processor is in an active state and its notify-CPU is processor  120 , so in box In box  950 , a guest running on the virtual processor may issue an instruction, such as a HLT or MWAIT instruction, to inform the VMM that it is idle, and give up its quantum of time. Therefore, in box  952 , the virtual processor may move into a halted state. 
     In box  954 , a notify event for the virtual processor, for example a physical interrupt from box  862  of method  800 , may be received by processor  120 . In box  956 , the VMM begin to service the physical interrupt, using the physical vector to identify the virtual processor for which the notify event was sent. In box  958 , the VMM may read the posted-interrupt descriptor for the virtual processor to determine the virtual interrupt request that is pending, according to the pIRR. In box  960 , the VMM may inject the virtual interrupt request into the virtual processor for servicing. In box  962 , the VMM may clear the Pending flag in the posted-interrupt descriptor. In box  964 , the virtual processor may be restarted on physical processor  120 . 
     In box  970 , the VMM may desire to migrate the virtual processor from physical processor  120  to physical processor  122  for load balancing or any other reason. Therefore, in box  972 , the VMM may cause a VM exit. In box  974 , the VMM may begin migrating the virtual processor from physical processor  120  to physical processor  122 . 
     In box  976 , the VMM may reprogram the notify event attributes in the posted-interrupt descriptor for the virtual processor. For example, the Dest-ID field of the posted-interrupt descriptor may be reprogrammed with the ID of the local APIC for processor  122 , and the Physical Vector field may be reprogrammed with the vector to be used by processor  122  to service the physical interrupt used as the posted-interrupt notify event. Therefore, embodiments of the present invention provide for migrating a virtual processor from one physical processor to another without reprogramming all of the virtual processor&#39;s IRTEs or other mapping information. 
     In box  978 , the migration of the virtual processor from physical processor  120  to physical processor  122  may end. In box  980 , the virtual processor may begin to run on physical processor  122 . 
     In box  998 , method  900  ends. 
     Within the scope of the present invention, methods  800  and  900  may be performed with illustrated boxes omitted, with additional boxes added, or with a combination of reordered, omitted, or additional boxes. 
     Thus, systems, apparatuses, and methods for posting interrupts to virtual processors have been disclosed. While certain embodiments have been described, and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art upon studying this disclosure. In an area of technology such as this, where growth is fast and further advancements are not easily foreseen, the disclosed embodiments may be readily modifiable in arrangement and detail as facilitated by enabling technological advancements without departing from the principles of the present disclosure or the scope of the accompanying claims.