On Thu, Oct 13, 2022, Vitaly Kuznetsov wrote:

Normally, genuine Hyper-V doesn't expose architectural invariant TSC
(CPUID.80000007H:EDX[8]) to its guests by default. A special PV MSR
(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x40000118) and corresponding CPUID
feature bit (CPUID.0x40000003.EAX[15]) were introduced. When bit 0 of the
PV MSR is set, invariant TSC bit starts to show up in CPUID. When the 
feature is exposed to Hyper-V guests, reenlightenment becomes unneeded.

Note: strictly speaking, KVM doesn't have to have the feature as exposing
raw invariant TSC bit (CPUID.80000007H:EDX[8]) also seems to work for
modern Windows versions. The feature is, however, tiny and straitforward
and gives additional flexibility so why not.

Vitaly Kuznetsov (7):
  x86/hyperv: Add HV_EXPOSE_INVARIANT_TSC define
  KVM: x86: Add a KVM-only leaf for CPUID_8000_0007_EDX
  KVM: x86: Hyper-V invariant TSC control
  KVM: selftests: Rename 'msr->available' to 'msr->fault_exepected' in
    hyperv_features test
  KVM: selftests: Convert hyperv_features test to using
    KVM_X86_CPU_FEATURE()
  KVM: selftests: Test that values written to Hyper-V MSRs are preserved
  KVM: selftests: Test Hyper-V invariant TSC control

For the series, in case Paolo ends up grabbing this:

Reviewed-by: Sean Christopherson <seanjc@google.com>