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Commit 09c9e813 authored by Glauber Costa's avatar Glauber Costa Committed by Avi Kivity
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KVM: Add cpuid.txt file 



This file documents cpuid bits used by KVM.

Signed-off-by: default avatarGlauber Costa <glommer@redhat.com>
Acked-by: default avatarZachary Amsden <zamsden@redhat.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 371bcf64

Documentation/kvm/cpuid.txt

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KVM CPUID bits

Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010

=====================================================



A guest running on a kvm host, can check some of its features using

cpuid. This is not always guaranteed to work, since userspace can

mask-out some, or even all KVM-related cpuid features before launching

a guest.



KVM cpuid functions are:



function: KVM_CPUID_SIGNATURE (0x40000000)

returns : eax = 0,

          ebx = 0x4b4d564b,

          ecx = 0x564b4d56,

          edx = 0x4d.

Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".

This function queries the presence of KVM cpuid leafs.





function: define KVM_CPUID_FEATURES (0x40000001)

returns : ebx, ecx, edx = 0

          eax = and OR'ed group of (1 << flag), where each flags is:





flag                               || value || meaning

=============================================================================

KVM_FEATURE_CLOCKSOURCE            ||     0 || kvmclock available at msrs

                                   ||       || 0x11 and 0x12.

------------------------------------------------------------------------------

KVM_FEATURE_NOP_IO_DELAY           ||     1 || not necessary to perform delays

                                   ||       || on PIO operations.

------------------------------------------------------------------------------

KVM_FEATURE_MMU_OP                 ||     2 || deprecated.

------------------------------------------------------------------------------

KVM_FEATURE_CLOCKSOURCE2           ||     3 || kvmclock available at msrs

                                   ||       || 0x4b564d00 and 0x4b564d01

------------------------------------------------------------------------------

KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side

                                   ||       || per-cpu warps are expected in

                                   ||       || kvmclock.

------------------------------------------------------------------------------