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_ASYNC_PF               ||     4 || async pf can be enabled by
                                   ||       || writing to msr 0x4b564d02
------------------------------------------------------------------------------
KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
                                   ||       || per-cpu warps are expected in
                                   ||       || kvmclock.
------------------------------------------------------------------------------
