WO2002052404A2

WO2002052404A2 - Processor mode for limiting the operation of guest software r unning on a virtual machine supported by a monitor

Info

Publication number: WO2002052404A2
Application number: PCT/US2001/045061
Authority: WO
Inventors: Stephen Chou; Gilbert Neiger; Erik Cota-Robles; Stalinselvaraj Jeyasingh; Richard Uhlig; Michael Kozuch; Alain Kagi
Original assignee: Intel Corporation (A Delaware Corporation)
Priority date: 2000-12-27
Filing date: 2001-11-27
Publication date: 2002-07-04
Also published as: WO2002052404A3; HK1058255A1; DE10197121B4; GB2386230A; BR0116599A; CN1561485A; RU2265880C2; KR20040028704A; TW594493B; GB2386230B; KR100602157B1; AU2002217992A1; US7818808B1; RU2003123118A; DE10197121T1; GB0314030D0; CN1295604C

Abstract

In one embodiment, a processor mode is provided for guest software. The processor mode enables the guest software to operate at a privilege level intended by the guest software. When the guest software attempts to perform an operation restricted by processor mode, the processor mode is exited to transfer control of the operation to a virtual-machine monitor, which runs outside this processor mode.

Description

NEW PROCESSOR MODE FOR LIMITING THE OPERATION OF GUEST

SOFTWARE RUNNING ON A VIRTUAL MACHINE SUPPORTED BY A

VIRTUAL MACHINE MONITOR

Field of the Invention

The present invention relates generally to virtual machines, and more

specifically to providing processor support for a virtual-machine monitor.

Background of the Invention A conventional virtual-machine monitor (VMM) typically runs on a computer and presents to other software the abstraction of one or more virtual

machines. Each virtual machine may function as a self-contained platform, running its own "guest operating system" (i.e., an operating system hosted by the

VMM). The guest operating system expects to operate as if it were running on a

dedicated computer rather than a virtual machine. That is, the guest operating system expects to control various computer operations and have access to hardware resources during these operations. The hardware resources may include processor-resident resources (e.g., control registers) and resources that

reside in memory (e.g., descriptor tables). However, in a virtual-machine

environment, the VMM should be able to have ultimate control over these

resources to provide proper operation of virtual machines and protection from and between virtual machines. To achieve this, the VMM typically intercepts and

arbitrates all accesses made by the guest operating system to the hardware

resources. Current implementations of VMMs may be based on software techniques for controlling access to hardware resources by the guest operating system. However, these software techniques may lack the ability to prevent guest

software from accessing some fields in the processor's control registers and memory. For instance, the guest operating system may not be prevented from

accessing a requestor privilege level (RPL) field in the code segment register of IA-32 microprocessors. In addition, existing software techniques typically suffer

from performance problems. Thus, an alternative mechanism is needed for supporting the operation of the NMM.

Brief Description of the Drawings

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like

reference numerals refer to similar elements and in which: Figure 1 illustrates one embodiment of a virtual-machine environment;

Figure 2 illustrates operation of a virtual-machine monitor based on guest deprivileging;

Figure 3 is a block diagram of a system for providing processor support to a virtual-machine monitor, according to one embodiment of the present invention;

Figure 4 is a flow diagram of a method for providing processor support to a virtual-machine monitor, according to one embodiment of the present invention;

Figure 5 is a flow diagram of a method for performing a transition out of N32 mode, according to one embodiment of the present invention;

Figure 6 is a flow diagram of a method for generating virtualization traps,

according to one embodiment of the present invention;

Figure 7 is a flow diagram of a method for mamtaining a redirection map, according to one embodiment of the present invention;

Figure 8 is a flow diagram of a method for controlling masking of interrupts,

according to one embodiment of the present invention; and

Figure 9 is a block diagram of one embodiment of a processing system. Description of Embodiments A method and apparatus for providing processor support to a virtual-machine monitor are described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one

skilled in the art that the present invention can be practiced without these specific details.

Some portions of the detailed descriptions that follow are presented in terms of algorithms and symbolic representations of operations on data bits

within a computer memory. These algorithmic descriptions and representations

are the means used by those skilled in the data processing arts to most effectively

convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to

a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined,

compared, and otherwise manipulated. It has proven convenient at times,

principally for reasons of common usage, to refer to these signals as bits, values,

elements, symbols, characters, terms, numbers, or the like. It should be borne in mind, however, that all of these and similar terms are

to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as "processing" or "computing" or "calculating" or " deter rnining" or "displaying" or the like, may refer to the action and processes of a computer system, or similar electronic computing

device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data

similarly represented as physical quantities within the computer-system

memories or registers or other such information storage, transmission or display

devices.

The present invention also relates to apparatus for performing the

operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general purpose computer selectively activated or

reconfigured by a computer program stored in the computer. Such a computer

program may be stored in a computer readable storage medium, such as, but is

not limited to, any type of disk including floppy disks, optical disks, CD-ROMs,

and magnetic-optical disks, read-only memories (ROMs), random access

memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer

system bus. instructions are executable using one or more processing devices

(e.g., processors, central processing units, etc.).

The algorithms and displays presented herein are not inherently related to

any particular computer or other apparatus. Various general-purpose machines

may be used with programs in accordance with the teachings herein, or it may

prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these machines will appear

from the description below. In addition, the present invention is not described with reference to any particular programrning language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein.

In the following detailed description of the embodiments, reference is made to the accompanying drawings that show, by way of illustration, specific

embodiments in which the invention may be practiced. In the drawings, like

numerals describe substantially similar components throughout the several views.

These embodiments are described in sufficient detail to enable those skilled in the

art to practice the invention. Other embodiments may be utilized and structural,

logical, and electrical changes may be made without departing from the scope of the present invention. Moreover, it is to be understood that the various

embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described

in one embodiment may be included within other embodiments. The following

detailed description is, therefore, not to be taken in a limiting sense, and the scope

of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. The method and apparatus of the present invention provide processor

support for a virtual-machine monitor (NMM). Figure 1 illustrates one

embodiment of a virtual-machine environment 100, in which the present

invention may operate. In this embodiment, bare platform hardware 116 comprises a computing platform, which may be capable, for example, of executing a standard operating system (OS) or a virtual-machine monitor (VMM), such as a VMM 112. A VMM, though typically implemented in software, may

export a bare machine interface, such as an emulation, to higher level software. Such higher level software may comprise a standard or real-time OS, although the

invention is not limited in scope in this respect and, alternatively, for example, a VMM may be run within, or on top of, another VMM. VMMs and their typical

features and functionality are well-known by those skilled in the art and may be implemented, for example, in software, firmware or by a combination of various

techniques.

As described above, a VMM presents to other software (i.e., "guest"

software) the abstraction of one or more virtual machines (VMs). Figure 1 shows

two VMs, 102 and 114. The guest software of each VM includes a guest OS such

as a guest OS 104 or 106 and various guest software applications 108-110. Each of the guest OSs 104 and 106 expects to control access to physical resources (e.g.,

processor registers, memory and memory-mapped I/O devices) within the

hardware platform on which the guest OS 104 or 106 is running and to perform

other functions. However, in a virtual-machine environment, the VMM 112

should be able to have ultimate control over the physical resources to provide

proper operation of VMs 102 and 112 and protection from and between VMs 102

and 114. The VMM 112 achieves this goal by intercepting all accesses of the guest OSs 104 and 106 to the computer's physical resources. Various techniques may be used to enable the VMM 112 to intercept the above accesses. One of such techniques is a guest-deprivileging technique which forces all guest software to

run at a hardware privilege level that does not allow that software access to certain hardware resources. As a result, whenever the guest OS 104 or 106 attempts to access any of these hardware resources, it "traps" to the VMM 112, i.e., the VMM 112 receives control over an operation initiated by the guest OS if

this operation involves accessing such hardware resources.

Figure 2 illustrates a prior art embodiment of the operation of a VMM that

supports guest deprivileging. As described above, guest deprivileging forces a

guest OS to execute in a less privileged mode of execution. For IA-32

microprocessors, the nature of page-based protection is such that all guest software runs at the least privileged level (i.e., ring 3). That is, a guest OS 206 and

guest applications 204 run at the same privilege level. As a result, the guest OS 206 may not be able to protect itself from the guest applications 206, thereby

possibly compromising the integrity of the guest OS 206. This problem is known

as ring compression.

Guest deprivileging may also cause an address-space compression problem. As described above, certain attempts of guest software to access hardware

resources result in traps that transfer control to the VMM 220. In order to enable

this transfer of control, a portion of VMM code and/ or data structures may be

architecturally required to reside in the same virtual-address space as the guest

OS 206. For instance, the IA-32 instruction-set architecture (ISA) may require that

an interrupt descriptor table (IDT) 212, a global descriptor table (GDT) 210 and

trap handling routines reside at the same virtual space as the guest OS 206. The VMM code and data structures 220 that reside in the virtual space 202 must be

protected from accesses by guest software (e.g., by running at ring 0). Accordingly, the guest OS 206 does not control the entire address space 202 as the guest OS 206 expects. This causes an address-space compression problem. Another limitation of VMMs that use guest deprivileging pertains to some cases in which the processors fail to prevent guest software from reading privileged hardware resources. For instance, the IA-32 microprocessors allow the

guest OS 206 to execute PUSH CS instructions which store a code segment register

into memory. One of this register's fields stores information about the current

privilege level. Accordingly, the guest OS 206 can become aware that its privilege level is 3, and not 0 as the guest OS 206 expects, by reading the value of the

current privilege level from the memory. As a result, the guest OS 206 may be exposed to the fact that it is running on a virtual machine, and the integrity of the

guest OS 206 may be compromised. Similarly, in some cases, the processors do not trap an attempt of the guest

software to modify privileged software resources. For instance, the IA-32

processors allow the guest OS 206 to issue POPF instructions which attempt to

load EFLAGS, and instead of generating a trap, simply ignore all or part of such attempts of the guest OS 206 because the guest OS 206 executes these instructions

with insufficient privilege. As a result, the guest OS 206 believes that a

corresponding EFLAGS field has been modified but the VMM 220 is not aware of

that and cannot properly emulate this modification. Accordingly, the guest OS 206 may be exposed to the fact that it is running on a virtual machine, and the integrity of the guest OS 206 may be compromised.

Yet another limitation of VM monitors that use guest deprivileging is caused

by excessive trapping. Because the number of hardware resource elements that need to be protected from accesses by guest software is significant and such

accesses may be frequent, traps may occur often. For instance, the IA-32

microprocessors support CLI instructions. The CLI instructions are issued to

modify an interrupt flag, which is an element of the privileged hardware resources and which thus cannot be accessed by unprivileged software. The guest

OS 206 commonly issues these instructions during its operation, thereby causing

frequent traps to the VMM 220. Frequent trapping negatively affects system

performance and reduces the utility of the VMM 220.

The present invention addresses the above problems and various other

limitations by providing processor support for a VMM. Figure 3 is block diagram of a system for providing processor support to a virtual-machine monitor,

according to one embodiment of the present invention.

Referring to Figure 3, all guest software runs at a processor mode referred to

herein as a virtual 32-bit mode (V32 mode). V32 mode allows the guest software

to run at its intended privilege level. For instance, for the IA-32 ISA, the guest OS

308 runs at the most privileged level (i.e., ring 0) and guest applications 306 run at

the least privileged level (i.e., ring 3). V32 mode restricts the operation of the guest software by preventing the guest software from performing operations that may result in its access of certain privileged hardware resources. V32 mode is exited when the guest software attempts to perform such an operation.

The VMM 320 runs outside V32 mode. When a transition out of V32 mode

occurs, the VMM 320 receives control over the operation initiated by the guest OS 308 or guest application 306. The VMM 320 then performs this operation, and

transfers control back to the guest software by entering V32 mode, thereby emulating the functionality desired by the guest software.

In one embodiment, V32 mode is implemented by mamtaining a flag in one of

the processor's control registers (e.g., CR0) to indicate whether the processor is in

V32 mode or not. In another embodiment, this flag (referred to herein as EFLAGSN32) is maintained in one of the reserved bits in the upper half of

EFLAGS. The EFLAGSN32 flag is modified either by a transition out of V32

mode or a transition into V32 mode.

In one embodiment, the ability of the processor to support V32 mode are

reported using one of the reserved feature bits that are returned in EDX when the

CPUID instruction is executed with the value 1 in EAX. It should be noted that a

variety of other mechanisms can be used to implement V32 mode and to report

the ability of the processor to support V32 mode without loss of generality.

In one embodiment, certain exceptions and interrupts cause a transition out of

V32 mode. These exceptions and interrupts include "virtualization traps." A

virtualization trap is generated when guest software that runs in V32 mode

attempts to perform an operation that may result in its access of certain privileged hardware resources. In one embodiment, when a transition out of V32 mode occurs, the guest address space 304 is automatically changed to the VMM address space 302. In addition, the processor state that was used by guest software is saved and stored in temporary registers, and the processor state required by the VMM 320 is loaded.

In one embodiment, when a transition into V32 mode occurs, the processor state that was saved on the transition out of V32 mode (i.e., to the VMM 320) is automatically restored, the VMM address space 302 is changed to the guest

address space 304, and control is returned to the guest OS 308.

In one embodiment, when guest software runs in V32 mode, software

interrupts (e.g., interrupts caused by execution of BOUND, INT or INTO instructions) are handled by the guest OS 308 using the guest IDT (i.e., the IDT

residing in the guest address space 304). All other interrupts and exceptions including virtualization traps cause a transition out of V32 mode which results in

a change of the guest address space 304 to the VMM address space 302. The IDT 316 is then used to point to code that handles a corresponding exception or

interrupt.

In one embodiment, a new interrupt flag (i.e., a virtual-machine interrupt flag)

is maintained for accesses by guest software. Whenever guest software attempts

to access the interrupt flag (IF), it will instead access the virtual machine interrupt flag (VMIF). In one embodiment, an attempt of guest software to access VMIF

(e.g., using the CLI instruction) does not cause a transition out of V32 mode,

except when the guest OS 308 has just set VMIF to 1 (e.g., through the STI instruction) and the VMM 320 wishes to deliver a pending interrupt to the guest OS 308. Such pending interrupts referred to herein as "virtual pending

interrupts" generate virtualization traps which allow the VMM 320 to deliver a pending interrupt to the guest software when the guest OS 308 signals that it is

ready to process such an interrupt. In one embodiment, one of the reserved bits in the upper half of the EFLAGS register is used to maintain a flag indicating

whether guest software has a pending virtual interrupt.

The implementation of V32 mode allows resolving all of the problems caused

guest deprivileging as described above. In particular, because guest software runs in V32 mode at its intended privilege level, the problem of ring compression

is eliminated. In addition, address-space compression is no longer a problem

because a virtualization trap automatically causes a switch to the VMM address

space 302, and therefore neither the tables controlling such transfers nor the code

handling a corresponding virtualization trap is required to reside in the guest

address space 304. Furthermore, because V32 mode enables the guest software to run at its

intended privilege level, the hardware resources that need to be protected no

longer include those elements of hardware resources that control the privilege

level. For instance, the PUSH CS instruction described above can no longer reveal

to the guest OS 308 that it runs on a virtual machine because the field of the code

segment register that stores information about a current privilege level now stores

the privilege level intended by the guest OS 308. Similarly, POPF instructions which attempt to load EFLAGS are no longer ignored when executed by the guest OS 308 because the guest OS 206 executes these instructions with sufficient privilege.

Accordingly, the number of elements of hardware resources that need to be protected is reduced. If any of them allow non- trapping read or write accesses by guest software, they are specifically architected to cause traps when executed in

V32 mode. Thus, the problems caused by non-trapping read and write accesses

are eliminated. In addition, because the implementation of V32 mode reduces the

number of elements of hardware resources that need to be protected, the number

of traps that occur when guest software attempts to access these elements is also

reduced. Frequency of traps is further reduced by providing mechanisms for eliminating traps caused by the most frequently used instructions. For instance,

STI instructions no longer cause traps except when guest software has a pending virtual interrupt.

Figure 4 is a flow diagram of a method 400 for providing processor support to

a virtual machine monitor, according to one embodiment of the present invention.

At processing block 404, guest software is executed in a processor mode (i.e., V32

mode) that allows guest software to operate at a privilege level intended by the

guest software. That is, a guest OS may operate at a supervisor privilege level,

and guest applications may operate at a user privilege level.

At processing block 406, an attempt of the guest software to perform an

operation restricted by V32 mode is identified. In response to this attempt, V32

mode is exited to transfer control over the operation initiated by the guest

software to the VMM which runs outside V32 mode (processor block 408). In one embodiment, the VMM configures what operations should cause a transition out of V32 mode as will be described in greater detail below in conjunction with Figure 7. In one embodiment, such operations generate virtualization traps that cause a transition out of V32 mode. Alternatively, any other mechanism known in the art can be used to cause a transition out of V32 mode. One embodiment of performing a transition out of V32 mode is described in greater detail below in conjunction with Figure 5.

Further, the VMM responds to the operation intended by the guest software

(processing block 410). Afterwards, V32 mode is re-entered to transfer control

over this operation back to the guest software (processing block 412), and method

400 returns to processing block 404. In one embodiment, when a transition into

V32 mode occurs, the processor state expected by the guest software is automatically restored and the VMM address space is changed to the guest

address space. Figure 5 is a flow diagram of a method 500 for performing a transition out of

V32 mode, according to one embodiment of the present invention. Method 500

begins with saving processor state used by guest software (processing block 504).

In one embodiment, the saved processor stated is stored in the processor's

temporary registers. At processing block 506, the processor state required by the VMM is loaded into processor registers. In one embodiment, loading the

processor state affects a change of the guest address space to the VMM address

space (e.g., the processor state is loaded by loading the control register CR3). In

an alternative embodiment, loading the processor state does not cause a change in the address space. In such an embodiment, at processing block 508, an address

space switch is performed to transfer from the guest address space to the VMM address space. Accordingly, when an interrupt or exception causing the

transition occurs, the IDT residing in the VMM address space is automatically used to point to the VMM-resident code for handling this interrupt or exception.

Figure 6 is a flow diagram of a method 600 for generating virtualization traps, according to one embodiment of the present invention. Method 600 begins with

identifying an attempt of guest software to perform an operation that may be restricted by V32 mode (processing block 604). At decision box 606, a

determination is made as to whether the attempt of the guest software can

potentially succeed. If the determination is positive, a virtualization trap is

generated (processing block 608). Alternatively, no virtualization trap is

generated, and the guest software proceeds with the operation (processing block

610). For instance, according to the IA-32 ISA, the RDMSR instruction can be executed only by software running with supervisor privilege. Consequently, if

the guest software OS which runs with supervisor privilege executes this

instruction, its attempt may be successful. If a guest application which runs with

user privilege executes this instruction, its attempt will not be successful, and a

general-protection fault will occur. Accordingly, an attempt of the guest OS to

execute the RDMSR instruction will cause a virtualization trap but an attempt of a

guest application will be handled by the guest OS.

In one embodiment, virtualization traps will be caused by potentially successful attempts of the guest OS to access the processor's control registers (e.g., CR0-CR4). For instance, for IA-32 processors, virtualization traps will be

generated in response to an attempt of the guest software to execute MOV CR (except the attempts to store CR2, which do not need to cause virtualization

traps), CLTS, LMSW or SMSW instructions, or a task switch. Virtualization traps may be also caused by a potentially successful attempt of the guest software to set

an interrupt flag IF (e.g., via STI, POPF or IRET instructions) if guest software has

a pending virtual interrupt. For IA-32 ISA, successful attempts to execute such

instructions as, for example, HLT, IN, INS/INSB/INSW/INSD, INVD, OUT,

OUTS/OUTSB/OUTSW/OUTSD, RDMSR, and WRMSR, will cause

virtualization traps. These virtualization traps will prevent guest software from halting the processor and from directly accessing I/O ports, caches or model-

specific registers. In addition, virtualization traps may be caused by attempts to execute CPUID instructions to allow the VMM to present the abstraction of

processor features chosen by the VMM, by attempts to execute INVLPG

instructions to enable the VMM to properly virtualize address translations, and by

attempts to execute IRET instructions (if IRET is used to transition into V32 mode)

used by guest software to implement a VMM to allow recursively nested VMMs. Figure 7 is a flow diagram of a method 700 for maintaining a redirection map,

according to one embodiment of the present invention. According to this

embodiment, the VMM maintains a redirection map to configure which interrupts

and exceptions should result in a virtualization trap (processing block 704). At

processing block 706, an occurrence of an interrupt or exception is identified. The redirection map is then consulted to find a bit associated with this interrupt or exception in the redirection bitmap (processing block 708).

At decision box 710, a determination is made as to whether this interrupt is

allowed to be handled by the guest OS. If the determination is positive, the interrupt or exception is delivered to V32 mode and is handled by the guest OS

(processing block 714). Alternatively, a virtualization trap is generated, causing a transition out of V32 mode (processing block 712).

Figure 8 is a flow diagram of a method 800 for controlling masking of interrupts, according to one embodiment of the present invention. Various

embodiments may be used to control the masking of interrupts. In one embodiment, all interrupts are unmasked when guest software is running. In this

embodiment, the guest software is permitted to manipulate an interrupt flag (e.g., for IA-32 microprocessors, this flag is identified as EFLAGS.IF), but this

manipulation will be ignored with respect to the masking of interrupts. In another embodiment, the masking of interrupts is dependent on the interrupt

flag. In this embodiment, the guest software is not permitted to manipulate the

interrupt flag. In particular, the guest software may be prevented from accessing

the interrupt flag by providing a shadow interrupt flag (e.g., EFLAGSNMIF) for

modifications by the guest software, by generating a virtualization trap in response to such an attempt of the guest software, or by using any other

technique known in the art.

Method 800 begins with identifying an attempt of guest software to modify

an interrupt flag that may potentially control masking of interrupts (processing block 804). At decision box 806, a determination is made as to whether the

interrupt flag controls the masking of interrupts. If the determination is negative, i.e., all interrupts are unmasked, the guest software is allowed to modify the interrupt flag (processing block 808). As described above, this modification will not have an effect on the masking of the interrupts.

Otherwise, if the masking of interrupts is dependent on the interrupt flag,

a determination is then made as to whether a shadow interrupt flag exists, i.e.,

whether the attempt of the guest software to affect the masking of interrupts is affecting the shadow flag (decision box 810). If the determination is negative, i.e.,

the guest software attempts to modify the actual interrupt flag, a virtualization

trap occurs (processing block 812), causing a transition out of N32 mode

(processing block 816). Alternatively, if the actual interrupt flag is not accessible to the guest software, the guest software is allowed to modify the shadow

interrupt flag (processing block 814). Figure 9 is a block diagram of one embodiment of a processing system.

Processing system 900 includes processor 920 and memory 930. Processor 920 can

be any type of processor capable of executing software, such as a microprocessor,

digital signal processor, microcontroller, or the like. Processing system 900 can be

a personal computer (PC), mainframe, handheld device, portable computer,

set-top box, or any other system that includes software.

Memory 930 can be a hard disk, a floppy disk, random access memory

(RAM), read only memory (ROM), flash memory, or any other type of machine medium readable by processor 920. Memory 930 can store instructions for performing the execution of the various method embodiments of the present

invention such as methods 400, 500, 600, 700 and 800 (Figures 4-8).

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description.

The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims

are entitled.

Claims

CLAIMSWhat is claimed is:

1. A method comprising: running guest software in a processor mode that enables the guest software to operate at a privilege level intended by the guest software; and responsive to an attempt of the guest software to perform an operation restricted by said processor mode, exiting said processor mode to transfer control

over the operation to the NMM running outside said processor mode.

2. The method of claim 1 further comprising: responding to the operation; and

transferring control over the operation to the guest software by entering said processor mode.

3. The method of claim 2 wherein entering said processor mode includes

loading processor state expected by the guest software.

4. The method of claim 1 wherein exiting said processor mode further comprises: saving processor state used by the guest software; and

loading processor state required by the NMM.

5. The method of claim 1 wherein exiting said processor mode further comprises automatically transferring from an address space associated with the guest software to an address space associated with the NMM.

6. The method of claim 1 further comprising maintaining a flag in a processor

control register to indicate whether the processor is in said processor mode.

7. The method of claim 1 further comprising reporting an ability of a processor to support said processor mode using one of a plurality of reserved

feature bits that are returned in a processor register.

8. The method of claim 1 wherein exiting said processor mode comprises generating one of a plurality of interrupts and exceptions in response to the

attempt of the guest software to perform the operation restricted by said processor mode.

9. The method of claim 8 wherein generating one of the plurality of interrupts

and exceptions further includes:

identifying the attempt of the guest software to perform the operation

restricted by said processor mode; and

deterrnining that the attempt of the guest software is potentially successful.

10. The method of claim 8 further comprising:

maintaining a redirection bitmap for the plurality of the interrupts and exception, the redirection bitmap indicating whether each of the plurality of the

interrupts and exceptions is allowed to be handled by the guest software; and consulting the redirection bitmap to determine whether to exit said processor mode.

11. The method of claim 8 further comprising:

identifying an attempt of the guest software to modify an interrupt flag;

and modifying the interrupt flag if the interrupt flag does not control masking of interrupts.

12. The method of claim 8 further comprising:

identifying an attempt of the guest software to modify an interrupt flag;

and preventing the attempt of the guest software to modify the interrupt flag.

13. The method of claim 12 wherein preventing the attempt of the guest

software to modify the interrupt flag includes providing a shadow interrupt flag

for modifications by the guest software.

14. The method of claim 12 wherein preventing the attempt of the guest software to modify the interrupt flag includes generating one of the plurality of interrupts and exceptions in response to the attempt of the guest software to

modify the interrupt flag.

15. A system comprising: a memory; and

a processor, coupled to the memory, to run guest software in a processor mode that enables the guest software to operate at a privilege level intended by

the guest software, to identify an attempt of the guest software to perform an

operation restricted by said processor mode, and to exit said processor mode, in

response to the attempt, to transfer control over the operation to a virtual-machine monitor (NMM) running outside said processor mode.

16. The system of claim 15 wherein the processor is to re-enter said processor

mode after the NMM responds to the operation.

17. The system of claim 16 wherein the processor is to load processor state

expected by the guest software when re-entering said processor mode.

18. The system of claim 15 wherein the processor is to save processor state

used by the guest software and to load processor state required by the NMM when exiting said processor mode.

19. The system of claim 15 wherein exiting said processor mode further comprises automatically transferring from an address space associated with the guest software to an address space associated with the NMM.

20. The system of claim 15 wherein the processor is to maintain a flag in a

processor control register to indicate whether the processor is in said processor mode.

21. The system of claim 15 wherein the processor is to reporting an ability to

support said processor mode using one of a plurality of reserved feature bits that

are returned in a processor register.

22. The system of claim 15 wherein the processor is to generate one of a plurality of interrupts and exceptions in response to the attempt of the guest

software to perform the operation restricted by said processor mode.

23. The system of claim 22 wherein the processor is to generate one of the

plurality of interrupts and exceptions upon determining that the attempt of the

guest software to perform the operation restricted by said processor mode is

potentially successful.

24. The system of claim 22 wherein the processor is to consult a redirection bitmap to determine whether to exit said processor mode, the redirection bitmap indicating whether each of the plurality of the interrupts and exceptions is allowed to be handled by the guest software.

25. The system of claim 22 wherein the processor is to identify an attempt of the guest software to modify an interrupt flag and to modify the interrupt flag if

the interrupt flag does not control masking of interrupts.

26. The system of claim 22 wherein the processor is to identify an attempt of the guest software to modify an interrupt flag and to prevent the attempt of the guest software to modify the interrupt flag.

27. The system of claim 26 wherein the processor is to prevent the attempt of

the guest software to modify the interrupt flag by providing a shadow interrupt

flag for modifications by the guest software.

28. A computer readable medium that provides instructions, which when

executed on a processor, cause said processor to perform operations comprising:

running guest software in a processor mode that enables the guest software

to operate at a privilege level intended by the guest software; and responsive to an attempt of the guest software to perform an operation restricted by said processor mode, exiting said processor mode to transfer control over the operation to the NMM running outside said processor mode.

29. The computer readable medium of claim 28 providing further instructions causing the processor to perform operations comprising: responding to the operation; and

30. The computer readable medium of claim 28 comprising further instructions

causing the processor to perform operations comprising: maintaining a redirection bitmap for the plurality of the interrupts and exception, the redirection bitmap indicating whether each of the plurality of the interrupts and exceptions is allowed to be handled by the guest software; and

consulting the redirection bitmap to determine whether to exit said

processor mode.