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.

Add the feature to KVM. Keep CPUID output intact when the feature
wasn't exposed to L1 and implement the required logic for hiding
invariant TSC when the feature was exposed and invariant TSC control
MSR wasn't written to. Copy genuine Hyper-V behavior and forbid to
disable the feature once it was enabled.

For the reference, for linux guests, support for the feature was added
in commit dce7cd62 ("x86/hyperv: Allow guests to enable InvariantTSC").
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221013095849.705943-4-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>