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A processor including a virtualization system of the processor with a memory virtualization support system to map a reference to guest-physical memory made by guest software executable on a virtual machine which in turn is executable on a host machine in which the processor is operable to a reference to host-physical memory of the host machine.
Virtualization enables a single host machine with hardware and software support for virtualization to present multiple abstractions of the host, such that the underlying hardware of the host machine appears as one or more independently operating virtual machines. Each virtual machine may therefore function as a self-contained platform. Often, virtualization technology is used to allow multiple guest operating systems and/or other guest software to coexist and execute apparently simultaneously and apparently independently on multiple virtual machines while actually physically executing on the same hardware platform. A virtual machine may mimic the hardware of the host machine or alternatively present a different hardware abstraction altogether.
Virtualization systems may include a virtual machine monitor (VMM) which controls the host machine. The VMM provides guest software operating in a virtual machine with a set of resources (e.g., processors, memory, IO devices). The VMM may map some or all of the components of a physical host machine into the virtual machine, and may create fully virtual components, emulated in software in the VMM, which are included in the virtual machine (e.g., virtual IO devices). The VMM may thus be said to provide a "virtual bare machine" interface to guest software. The VMM uses facilities in a hardware virtualization architecture to provide services to a virtual machine and to provide protection from and between multiple virtual machines executing on the host machine.
As guest software executes in a virtual machine, certain instructions executed by the guest software (e.g., instructions accessing peripheral devices) would normally directly access hardware, were the guest software executing directly on a hardware platform. In a virtualization system supported by a VMM, these instructions may cause a transition to the VMM, referred to herein as a virtual machine exit. The VMM handles these instructions in software in a manner suitable for the host machine hardware and host machine peripheral devices consistent with the virtual machines on which the guest software is executing. Similarly, certain interrupts and exceptions generated in the host machine may need to be intercepted and managed by the VMM or adapted for the guest software by the VMM before being passed on to the guest software for servicing. The VMM then transitions control to the guest software and the virtual machine resumes operation. The transition from the VMM to the guest software is referred to herein as a virtual machine Entry.
As is well known, a process executing on a machine on most operating systems may use a virtual address space, which is an abstraction of the underlying physical memory system. As is known in the art, the term virtual when used in the context of memory management e.g. "virtual address," "virtual address space," "virtual memory address" or "virtual memory space," refers to the well-known technique of a processor based system, generally in conjunction with an operating system, presenting an abstraction of underlying physical memory to a process executing on a processor-based system. For example, a process may access a virtual, contiguous and linearized address space abstraction which is mapped to non-linear and non-contiguous physical memory by the underlying operating system. This use of virtual is distinguishable from the use of the same term used in the context virtualization, where virtual generally refers to an abstraction that simulates a physical machine e.g. "virtual machine," "virtual bare machine," "virtual hardware," "virtual processor" or "virtual network interface." The intended meaning of the term will be clear to one in the art based on the context in which it is used herein.
FIG. 1?shows a process executing on a processor-based system which incorporates a processor and a memory communicatively coupled to the processor by a bus. With reference to?FIG. 1, when a process?105?references a memory location?110?in its virtual address space?115?(process virtual memory space), a reference to an actual address?140?in the physical memory?145?of the machine?125?(machine physical memory) is generated by memory management130, which may be implemented in hardware (sometimes incorporated into the processor?120) and software (generally in the operating system of the machine). Memory management?130, among other functions maps a location in the virtual address space to a location in physical memory of the machine. As shown in FIG. 1, a process may have a different view of memory from the actual memory available in the physical machine. In the example depicted in?FIG. 1, the process operates in a virtual address space from 0 to 1 MB which is actually mapped by the memory management hardware and software into a portion of the physical memory which itself has an address space from 10 to 11 MB; to compute a physical address from a process space address, an offset?135?may be added to the process virtual address. More complex mappings from process virtual memory space to physical memory are possible, for example, the physical memory corresponding to process virtual memory may be divided into parts such as pages and be interleaved with pages from other processes in physical memory.
Memory is customarily divided into pages, each page containing a known amount of data, varying across implementations, e.g. a page may contain 4096 bytes of memory. As memory locations are referenced by the executing process, they are translated into page references. In a typical machine, memory management maps a reference to a page in process virtual memory to a page in machine physical memory. In general, memory management may use a page table to specify the physical page location corresponding to a process space page location.
One aspect of managing guest software in a virtual machine environment is the management of memory. Handling memory management actions taken by the guest software executing in a virtual machine creates complexity for a controlling system such as a virtual machine monitor. Consider for example a system in which two virtual machines execute via virtualization on a host machine implemented on a 32-bit IA-32 Intel(& Architecture platform (IA-32), which is described in the IA-32 Intel? Architecture Software Developer‘s Manual (IA-32 documentation). The IA-32 platform may include IA-32 page tables implemented as part of an IA-32 processor. Further, assume that each virtual machine itself presents an abstraction of an IA-32 machine to the guest software executing thereon. Guest software executing on each virtual machine may make references to a guest process virtual memory address, which in turn is translated by the guest machine‘s memory management system to a guest-physical memory address. However, guest-physical memory itself may be implemented by a further mapping in host-physical memory through a VMM and the virtualization subsystem in hardware on the host processor. Thus, references to guest memory by guest processes or the guest operating system, including for example references to guest IA-32 page table control registers, must then be intercepted by the VMM because they cannot be directly passed on to the host machine‘s IA-32 page table without further reprocessing, as the guest-physical memory does not, in fact, correspond directly to host-physical memory but is rather further remapped through the virtualization system of the host machine.
FIG. 1?depicts the relationship between process and physical memory (Prior Art).
FIG. 2?depicts abstractly the relationship between virtual machines and a host machine in one embodiment.
FIG. 3?depicts a high level structure of a virtual machine environment in one embodiment.
FIGS. 4?a?and?4?b?illustrate processing in one embodiment of a virtual machine environment.
FIG. 5?depicts address computation using extended paging tables in one embodiment.
FIG. 6?depicts address computation using hierarchical extended paging tables in one embodiment.
FIG. 7?depicts an extended paging table base address pointer in one embodiment.
FIG. 8?depicts an extended paging table entry in one embodiment.
FIG. 2:?FIG. 2?depicts the relationship between one or more virtual machines executing on a host machine with specific regard to the mapping of guest memory in one embodiment.?FIG. 2?illustrates how guest-physical memory is remapped through the virtualization system of the host machine. Each virtual machine such as virtual machine A,?242, and virtual machine B,?257, presents a virtual processor?245?and?255?respectively to guest software running on the virtual machines. Each machine provides an abstraction of physical memory to the guest operating system or other guest software, guest-physical memories?240?and250, respectively. As guest software executes on the virtual machines?242?and?257, it is actually executed by the host machine?267?on host processor?265?utilizing host-physical memory?260.
As shown in?FIG. 2, in this embodiment, guest-physical memory?240?which is presented as a physical memory space starting at address 0 in virtual machine A,?242, is mapped to some contiguous region?270?in host-physical memory?260. Similarly, guest-physical memory?250?in virtual machine B,?257, is mapped to a different portion?275?of host-physical memory?260. As shown in?FIG. 2, the host machine might have 1024 MB of host-physical memory. If each virtual machine242?and?257?is assigned 256 MB of memory, one possible mapping might be that virtual machine A,?242, is assigned the range 128-384 MB and virtual machine B,?257, is assigned the range 512-768 MB. Both virtual machines?242?and?257reference a guest-physical address space of 0-256 MB. Only the VMM is aware that each virtual machine‘s address space maps to different portions of the host-physical address space.
The virtual machines and memory mapping shown in?FIG. 2?are only one representation of one embodiment, in other embodiments, the actual number of virtual machines executing on a host machine may vary from one to many; the actual memory sizes of the host machine and the virtual machines may vary and be variable from virtual machine to virtual machine. The example depicts a simple, contiguous allocation of memory to virtual machines. In a more general case, the physical-memory pages allocated to a virtual machine may not be contiguous and might be distributed in the host-physical memory interleaved with each other and with pages belonging to the VMM and to other host processes.
A processor-based system that is presented as a virtual machine in a system such as that depicted in?FIG. 2?may implement a virtual machine in all its complexity. Thus for example, a virtual machine may present a full view of guest-physical memory to the guest OS, and perform memory management for guest software executing on the virtual machine, using memory management provided by the guest OS and the virtual processor or other virtual hardware of the virtual machine. In one exemplary embodiment, the virtual machine may present an IA-32 platform including IA-32 hardware support such as page tables for memory management to the guest OS, and in turn be actually executing on a host platform which is also an IA-32 platform including IA-32 hardware for memory management. Without additional mechanisms, a virtualization system in this embodiment must implement a physical-memory virtualization algorithm in the VMM using, as one possible solution, IA-32 page table shadowing to remap, partition and protect physical memory. Thus, for example, when guest software attempts to access the IA-32 page tables of the virtual machine, the VMM must overlay functionality required for virtualization (e.g., remapping physical addresses) onto the functionality required by the guest OS.
To this end, the VMM must trap a variety of events surrounding the use of the paging mechanism by the guest software. This includes writes to control registers such as control registers of the IA-32 memory management system (e.g., CR0, CR3?and C4), accesses to model-specific registers-(MSRs) associated with paging and memory access (e.g., memory-type range registers (MTRRs)), handling certain exceptions (e.g., page faults), as described in the IA-32 documentation. This use of the IA-32 page tables to virtualize physical memory is complex and exacts a significant performance overhead.
FIG. 3:?FIG. 3?illustrates one embodiment of a virtual-machine environment?300. In this embodiment, a processor-based platform?316?may execute a VMM?312. The VMM, though typically implemented in software, may emulate and export a virtual bare machine interface to higher level software. Such higher level software may comprise a standard OS, a real time OS, or may be a stripped-down environment with limited operating system functionality and may not include OS facilities typically available in a standard OS in some embodiments. Alternatively, for example, the VMM?312?may be run within, or using the services of, another VMM. VMMs may be implemented, for example, in hardware, software, firmware or by a combination of various techniques in some embodiments.
The platform hardware?316?may be a personal computer (PC), mainframe, handheld device such as a personal digital assistant (PDA) or "smart" mobile phone, portable computer, set top box, or another processor-based system. The platform hardware?316?includes at least a processor?318?and memory?320. Processor?318?may be any type of processor capable of executing programs, such as a microprocessor, digital signal processor, microcontroller, or the like. The processor may include microcode, programmable logic or hard coded logic for execution in embodiments. Although?FIG. 3shows only one such processor?318, there may be one or more processors in the system in an embodiment. Additionally, processor?318?may include multiple cores, support for multiple threads, or the like. Memory?320?can comprise a hard disk, a floppy disk, random access memory (RAM), read only memory (ROM), flash memory, any combination of the above devices, or any other type of machine medium readable by processor?318?in various embodiments. Memory?320?may store instructions and/or data for performing program execution and other method embodiments.
The VMM?312?presents to guest software an abstraction of one or more virtual machines, which may provide the same or different abstractions to the various guests.?FIG. 3?shows two virtual machines,?302?and?314. Guest software such as guest software?303?and?313?running on each virtual machine may include a guest OS such as a guest OS?304?or?306?and various guest software applications?308?and?310. Guest software?303?and?313?may access physical resources (e.g., processor registers, memory and I/O devices) within the virtual machines on which the guest software?303?and?313?is running and to perform other functions. For example, the guest software?303?and?313?expects to have access to all registers, caches, structures, I/O devices, memory and the like, according to the architecture of the processor and platform presented in the virtual machine?302?and?314.
In one embodiment, the processor?318?controls the operation of the virtual machines?302?and?314?in accordance with data stored in a virtual machine control structure (VMCS)?324. The?VMCS?324?is a structure that may contain state of guest software?303?and?313, state of the VMM?312, execution control information indicating how the VMM?312?wishes to control operation of guest software?303?and?313, information controlling transitions between the VMM?312?and a virtual machine, etc. The processor?318?reads information from the?VMCS?324?to determine the execution environment of the virtual machine and to constrain its behavior. In one embodiment, the?VMCS?324?is stored in memory?320. In some embodiments, multiple VMCS?structures are used to support multiple virtual machines.
The VMM?312?may need to manage the physical memory accessible by guest software running in the virtual machines?302and?314. To support physical memory management in one embodiment, the processor?318?provides an extended page table (EPT) mechanism. In the embodiment, the VMM?312?may include a physical memory management module?326?that provides values for fields associated with physical memory virtualization that may need to be provided before transition of control to the virtual machine?302?or?314. These fields are collectively referred to as?EPT?controls.?EPT?controls may include, for example, an?EPT?enable indicator specifying whether the?EPT?mechanism should be enabled and one or more EPT?table configuration controls indicating the form and semantics of the physical memory virtualization mechanism. These will be discussed in detail below. Additionally, in one embodiment,?EPT?tables?328?indicate the physical address translation and protection semantics which the VMM?312?may place on guest software?303?and?313.
In one embodiment, the?EPT?controls are stored in the?VMCS?324. Alternatively, the?EPT?controls may reside in a processor?318, a combination of the memory?320?and the processor?318, or in any other storage location or locations. In one embodiment, separate?EPT?controls are maintained for each of the virtual machines?302?and?314. Alternatively, the same?EPT?controls are maintained for both virtual machines and are updated by the VMM?312?before each virtual machine entry.
In one embodiment, the?EPT?tables?328?are stored in memory?320. Alternatively, the?EPT?tables?328?may reside in the processor?318, a combination of the memory?320?and the processor?318, or in any other storage location or locations. In one embodiment, separate?EPT?tables?328?are maintained for each of the virtual machines?302?and?314. Alternatively, the same?EPT?tables?328?are maintained for both virtual machines?302?and?314?and are updated by the VMM?312?before each virtual machine entry.
In one embodiment, the processor?318?includes?EPT?access logic?322?that is responsible for determining whether the?EPT mechanism is enabled based on the?EPT?enable indicator. If the?EPT?mechanism is enabled, the processor translates guest-physical addresses to host-physical addresses-based on the?EPT?controls and?EPT?tables?328.
In one embodiment, in which the system?300?includes multiple processors or multi-threaded processors, each of the logical processors is associated with a separate?EPT?access logic?322, and the VMM?312?configures the?EPT?tables?328?and?EPT controls for each of the logical processors.
Resources that can be accessed by guest software (e.g.,?303, including guest OS?304?and application?308) may either be classified as "privileged" or "non-privileged." For privileged resources, the VMM?312?facilitates functionality desired by guest software while retaining ultimate control over these privileged resources. Further, each guest software?303?and?313?expects to handle various platform events such as exceptions (e.g., page faults, general protection faults, etc.), interrupts (e.g., hardware interrupts, software interrupts), and platform events (e.g., initialization (INIT) and system management interrupts (SMIs)). Some of these platform events are "privileged" because they must be handled by the VMM?312?to ensure proper operation of virtual machines?302?and?314?and for protection from and among guest software. Both guest operating system and guest applications may attempt to access privileged resources and both may cause or experience privileged events. Privileged platform events and access attempts to privileged resources are collectively referred to as "privileged events‘ or ‘virtualization events" herein.
FIGS. 4?a?and?4?b:?Operation of a virtual machine environment in an embodiment such as that previously described and depicted in?FIG. 3?is depicted by processing shown in?FIGS. 4?a?and?4?b.?FIG. 4?a?depicts the operation of a VM environment in an embodiment to process a privileged event occurring in guest software; and the operation of the embodiment to process a non-privileged event by guest software.?FIG. 4?b?depicts operations of a VM environment in an embodiment specifically related to extended paging tables, specifically relating to guest software access to guest-physical memory and to the management of the?EPT?mechanism in hardware by the VMM in the embodiment.?FIGS. 4?a?and?4?b?do not depict all components or all operations that may occur in an environment such as that depicted in?FIG. 3. This is solely for clarity of presentation. While a small set of components and a few specific operations are represented in?FIGS. 4?a?and4?b,?a VM environment in an embodiment may comprise many other components, and many other operations may take place in such an embodiment.
FIG. 4?a?is considered first.?FIG. 4?a?depicts one exemplary set of operations of guest software?303?executing on a virtual machine abstraction?302, and platform hardware?316?previously described in?FIG. 3. The operations are depicted within blocks indicating where in the system (e.g. in the VMM?312, in the guest software?303, etc.) they occur. In addition to other components of the VM environment previously described, VM abstraction?302?may store a virtual machine state and other state information for the guest software?303?at?412?and may also provide other resources such as a virtual network connection or set of general registers, to name two of many examples, to guests. Of course, the physical resources that implement VM state, guest state, and other VM resources are actually provided by the platform hardware?316?on which the VM executes. The platform hardware includes memory?320,?VMCS?324?and processor?318.
At?440, guest software?303?accesses a non-privileged resource?442. Non-privileged resources do not need to be controlled by the VMM?312?and can be accessed directly by guest software which continues without invoking the VMM?312, allowing the guest to continue operation at?445?after accessing the non-privileged resource?442. A non-privileged platform event would likewise be handled without the intervention of the VMM?312?(this is not shown in?FIG. 4?a).
At?405, the guest software?303?attempts to access a privileged resource, and/or experiences a privileged platform event. When such a privileged event occurs as at?405, control may be transferred?407?to the VMM?312. The transfer of control407?from guest software to the VMM?312?is referred to herein as a virtual machine exit. After facilitating the resource access or otherwise handling the privileged event appropriately, the VMM?312?may return control to guest software as at432?which then resumes operation,?435. The transfer of control?432?from the VMM?312?to guest software is referred to as a virtual machine entry. In one embodiment, the VMM?312?initiates a virtual machine entry by executing an instruction specially designed to trigger the transition,?430, referred to herein as a virtual machine entry instruction.
In one embodiment, when a virtual machine exit occurs, components of the processor state used by guest software are saved,?410, components of the processor state required by the VMM?312?are loaded, and the execution resumes in the VMM?312?at?420. In one embodiment, the components of the processor state used by guest software are stored in a guest-state area of?VMCS?324?and the components of the processor state required by the VMM?312?are stored in a monitor-state area of?VMCS?324. In one embodiment, when a transition from the VMM?312?to guest software occurs, components of the processor state that were saved at the virtual machine exit (and may have been modified by the VMM312?while processing the virtual machine exit) are restored?425?and control is returned to the virtual machine?302?or?314?at430.
Next,?FIG. 4?b?is considered. As noted previously,?FIG. 4?b?depicts those operations of the VM environment described above and depicted in?FIG. 4?a?specifically related to extended paging tables, to guest program access to guest-physical memory and to the management of the?EPT?mechanism in hardware by the VMM in one embodiment. As before, for clarity of presentation?FIG. 4?b?does not depict all components or all operations that may occur in a VM environment in an embodiment. While a small set of components and a few specific operations are represented in?FIG. 4?b,?a VM environment in an embodiment may comprise many other components, and many other operations may take place in such an embodiment.
The components of the VM environment in the embodiment depicted in?FIG. 4?b?are the guest software?303, VM?302, VMM312?with a physical memory management module?326, and platform hardware or physical machine?316. The platform hardware further comprises memory?320, including, in this embodiment, a set of?EPT?tables?328?and a?VMCS?324; and a processor?318?with?EPT?access logic?322. In general a use of the?EPT?facilities in platform hardware may be initiated by guest software, as shown in?FIG. 4?at?450, when an access to guest-physical memory is made, for instance by the guest software?303. Guest-physical memory accesses are referred to the VM abstraction of memory?451?provided by VM?302, which in turn is referred to the physical machine?316. If the?EPT?mechanism is enabled, the platform hardware?316?may process the VM reference to guest-physical memory using the?EPT?access logic?322?and the?EPT?tables?328?to translate an access to guest-physical memory to an access to host-physical memory?320. Details of?EPT?operation are discussed with reference to?FIGS. 5 and 6?below
The?EPT?mechanism itself may be configured by the VMM?312?which configures the?EPT?tables?328?and the?EPT?controls which may be stored in the?VMCS?324. In this embodiment, the configuration of the?EPT?mechanism may be done by the VMM?312?as part of the operation of the physical memory management module?326?following the processing of a privileged event?405?in the VMM?312?and prior to VM entry?430. In configuring the?EPT?mechanism, the VMM?312?may update theEPT?tables?328?and?EPT?controls, in order to enable, disable or otherwise control the?EPT?mechanism,?460.
Of course, many other forms of processing are possible for the use of extended paging tables in conjunction with a VM environment, for example, different locations for the?EPT?controls and?EPT?tables?328?as discussed earlier with reference toFIG. 3, multiple VMs, multiple processors, multiple threads, multiple guests, and combinations of these variations, among many others.
FIG. 5:?FIG. 5?shows one example of processing using the extended page tables introduced above to ultimately compute a host-physical address when guest software in a virtual machine references a guest virtual address. The example depicted shows guest software running in an IA-32 platform using simple 32-bit virtual addressing and simple page table formats. One skilled in the art will easily be able to extend this example to understand, for example, other paging modes (e.g., 64-bit addressing in the guest software), other instruction set architectures (e.g., The Intel Itanium (Architecture, as specified, for example, in the Intel Itanium Architecture Software Developer‘s Manual, available from Intel Corporation) or other configurations.
In?FIG. 5 a?reference to a guest virtual address?510?is executed by guest software executing in a virtual machine. The memory management mechanism active in the guest (i.e., configured by the guest operating system) is used to translate the virtual address to a guest-physical address. Each guest-physical address used in the translation, and the resulting guest-physical address, are translated to host-physical addresses through?EPT?before accessing the host-physical memory. This process is detailed in the following discussion.
In this example, the appropriate bits?502?in the CR3?register?520?point to the base of the guest‘s page directory table?560?in guest-physical memory. This value?502?is combined with the upper bits from the guest virtual address?510?(appropriately adjusted, according to IA-32 semantics by multiplying by 4 because, in this example, the entries in the tables are 4 bytes each) to form the guest-physical address?512?of the page directory entry (PDE) in the guest‘s PD table?560. This value?512is translated through the?EPT?tables?555?to form the host-physical address?504?of the page directory entry. The processor accesses the page directory entry using this host-physical address?504.
Information from the PDE includes the base address?522?of the guest‘s page table?570. This guest-physical address?522?is combined with bits?21:12?of the guest virtual address?510?appropriately adjusted to form the guest-physical address?532?of the page table entry in the guest‘s page table?570. This guest-physical address?532?is translated through the?EPT?tables565?to form the host-physical address?514?of the guest‘s page table entry (PTE). The processor accesses the PTE using this host-physical address?514.
Information from the PTE includes the base address?542?of the page in guest-physical memory being accessed. This value is combined with the low-order bits (11:0) of the guest virtual address?510?to form the guest-physical address?552?of the memory being accessed. This value?552?is translated through the?EPT?tables?575?to form the host-physical address?524?of the memory being accessed.
Each time the?EPT?tables are used to translate a guest-physical address to a host-physical address, the processor also validates that the access is permitted according to controls in the?EPT?tables, as will be described below. Additionally, it must be understood that the?EPT?tables?555,?565, and?575, though indicated as distinct in?FIG. 5?may, in one embodiment, be the same set of?EPT?tables (i.e., a single set of?EPT?tables is used for all address translations from guest-physical to host-physical).
FIG. 6:?FIG. 6?depicts another example of processing using the extended page tables introduced above to ultimately translate a guest-physical address to a host-physical address using a multi-leveled?EPT?table. In the exemplary embodiment shown in?FIG. 6, the appropriate bits?602?in the in an?EPT?base pointer (EPTP)?620?indicate the host-physical address of the base of the first-level?EPT?table?650, which in this embodiment is stored in host-physical memory. The EPTP will be discussed in more detail below with regard to?FIG. 7. In this example, the entries in the?EPT?tables are 8 bytes each. Bits38:30?from the guest-physical address?610?(601) are appropriately adjusted by multiplying by 8 (for example, by shifting the value left by 3 bits) to obtain an adjusted upper guest-physical address?603. The?EPT?table base address value?602?is combined with (added to) the adjusted upper guest-physical address bits?603, forming the host-physical address?604?of an EPT?table entry?651?in a first level?EPT?table?650. An exemplary format of an entry such as?651?in the first level?EPT?table650?as well as entries in the other?EPT?tables?660?and?670?will be discussed below with regard to?FIG. 8.
Part of the?EPT?table entry?651?is the base address?612?of the next-level?EPT?table?660. A second adjusted address component?613?is formed from bits?29:21?(611) of the guest-physical address?610. This adjusted value?613?is combined with (added to) the base address?612?to form the host-physical address?614?of an?EPT?table entry?661?in the next level EPT?table?660. The processor accesses the?EPT?table entry?661?using this host-physical address?614.
Information from the?EPT?table entry?661?includes the base address?622?of the final?EPT?table?670. This base address?622is combined with adjusted bits?20:12?(623) of the guest virtual address?610?after appropriately adjusting to form the address624?of the?EPT?table entry?671?in the final?EPT?table?670. The processor accesses the?EPT?table entry using this host-physical address?624.
Information from the?EPT?table entry?671?includes the base address?632?of the page being accessed in host-physical memory?690. This page address value?690?is combined with the low-order bits (11:0) of the guest-physical address?610?to form the final host-physical address?634?of the memory being accessed.
In the exemplary embodiment shown in?FIG. 6, the?EPT?tables are hierarchical. They are similar in form to traditional multi-level page tables. Furthermore, in this example, each?EPT?table entry in each?EPT?table is 8 bytes in size, though in other embodiments this size may be different, changing the mechanics of accessing the tables as would be appreciated by one in the art. In this example, each?EPT?table is 4 KB in size. In other embodiments, different table sizes may be used; moreover, it is not required that all tables in a hierarchy like that depicted in?FIG. 6?should be of the same size. This change in size may impact the number of bits used from the guest-physical address to index the next level of the?EPT?table. It will be obvious to one in the art that many other?EPT?table configurations are possible.
The hierarchical configuration depicted in the figure shows three levels of hierarchy, with two of the?EPT?tables?650?and?660serving as indices to lower level?EPT?tables?660?and?670?respectively. In other embodiments, there may be fewer, e.g. two levels, or more e.g. four or more, levels of hierarchy in such a hierarchical table. In general the number of levels of hierarchy may vary depending at least in part on one or more of the number of bits in the guest-physical address, the size of each table, and the number of bytes in each table entry. The guest-physical address in the example in?FIG. 6?is 32 bits in size. In other embodiments, the guest-physical address may be a different size; this change in size may require a change in the number of levels of?EPT?table that are required to perform a translation. For example, if the guest-physical address is 48 bits, 4 levels of?EPT?tables are required to do the translation (assuming?4KB?EPT?tables at each level and 8-byte?EPT?table entries in each?EPT?table).
In the embodiment shown in?FIG. 6, the?EPT?controls include a single field, the?EPT?pointer (EPTP). This field contains the address of the base of the first level?EPT?table. In this example, each?EPT?table is?4KB is size.
FIG. 7: As shown in an exemplary embodiment depicted in?FIG. 7, the?EPT?base address pointer (EPTP) includes bits used to form the base address (in host-physical memory) of the base of the first level?EPT?table such as that described above in FIG. 6. In the example depicted in?FIG. 7, bits?59:12?form the base address. Bits?11:0?and?63:60?are assumed to be 0. Of course, the widths of the various bit fields may vary in other embodiments, for example the base address field will change depending on the number of address bits in a particular architecture or implementation. The remaining bits in the EPTP register may be used for other purposes in other embodiments. In one embodiment, the EPTP register is accessible only through a virtual machine entry or virtual machine exit. In such an embodiment, the EPTP register in the processor is loaded from an EPTP field in the?VMCS?at the time of a virtual machine entry, activating the?EPT?mechanism while the guest software operates. As indicated above, this activation (and loading of the EPTP field) may be controlled by other control bits within the?VMCS?or elsewhere.
FIG. 8: This figure depicts an exemplary embodiment of the format of an entry in an?EPT?table. In this example, each entry in an?EPT?table is 8 bytes in size. In one embodiment, each?EPT?table is 4 KB in size, meaning that there are 512?EPT?table entries per?EPT?table page. As shown in the example in?FIG. 8, each?EPT?table entry contain the base host-physical address of the next level?EPT?table or page in memory (ADDR) and permission and other configuration information. As before, the widths of the various bit fields may vary in other embodiments, for example the ADDR width may change depending on the number of address bits in a particular architecture or implementation.?FIG. 8?depicts only 2 permission bits, Present and Writable. In other embodiments, other permission and configuration information may be present in each EPT?table entry. For example, in one embodiment, a permission bit indicates if a page of memory may be executed (i.e., if the contents of the page may be fetched and interpreted as instructions by the processor)
The?EPT?tables may be in a variety of different formats. For example, they may be implemented as shown in?FIG. 6?as simple, hierarchical tables. Alternatively, they may be single level page tables (where the size of the first level?EPT?table dictates the maximum size of the guest-physical address space). Alternatively, they may be hashed tables in some form. It will be obvious to one skilled in the art that a myriad of possible configurations are possible in other embodiments.
The?EPT?tables may support one or more sizes of pages in host-physical memory. In one embodiment, each entry in each EPT?table includes a super page bit which indicates that the walk of the?EPT?tables should stop at that point and the host-physical address formed using the address information in the?EPT?table entry and the remaining bits in the guest-physical address. In the example shown in?FIG. 6, for example, if a super page bit was set in the?EPT?tables?660, the resulting page in host-physical memory would be?2?MB in size and the resulting host-physical address would be formed by combining bits20:0?of the guest-physical address?610?with the address bits from the?EPT?table?660.
In some embodiments, the extended paging tables and?EPT?address translation mechanism may be enabled by virtual machine entry and disabled by virtual machine exit. Thus, as a consequence, the?EPT?mechanism may not be available for use by either guest software or VMM software to manage its own address translation. Furthermore, in such embodiments, the?EPT?mechanism may be distinct from and independent of other conventional memory page management mechanisms available to guest or host software, such as for example the IA-32 paging tables in an IA-32 embodiment, though?EPT operations may utilize features of the conventional page management mechanism. Thus, the organization and operation of the?EPT?tables may be completely distinct from other page translation facilities provided by the processor for conventional execution of programs and operations directly on the host machine, as opposed to the execution of guest software that utilizes the virtualization and?EPT?mechanisms of the host machine. In one embodiment, the?EPT?mechanism may utilize tables in the same format as that used by a conventional page management mechanism of the embodiment, available to guest and VMM software. However, the tables controlling the?EPT?mechanism may still be distinct from those controlling either guest-virtual address to guest-physical address translation and from those controlling host-virtual address to host-physical address translation.
Although the examples provided may describe providing support for physical memory virtualization in a virtual machine system in the context of execution units and logic circuits, other embodiments can be accomplished by way of software. Some embodiments may be provided as a software program product or software which may include a machine or machine-readable medium having stored thereon instructions which when accessed by the machine perform a process of the embodiment. In other embodiments, processes might be performed by specific hardware components that contain hardwired logic for performing the processes, or by any combination of programmed components and custom hardware components.
SRC=https://www.google.com.hk/patents/US20060161719
Virtualizing physical memory in a virtual machine system
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原文地址：http://www.cnblogs.com/coryxie/p/3960309.html
