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 = 0x40000001,
          ebx = 0x4b4d564b,
          ecx = 0x564b4d56,
          edx = 0x4d.
Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
The value in eax corresponds to the maximum cpuid function present in this leaf,
and will be updated if more functions are added in the future.
Note also that old hosts set eax value to 0x0. This should
be interpreted as if the value was 0x40000001.
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_PV_UNHALT              ||     7 || guest checks this feature bit
                                   ||       || before enabling paravirtualized
                                   ||       || spinlock support.
------------------------------------------------------------------------------
KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
                                   ||       || per-cpu warps are expected in
                                   ||       || kvmclock.
------------------------------------------------------------------------------