Re: [PATCH v3 3/6] KVM: x86: Hyper-V invariant TSC control

[Sean Christopherson]

On Wed, Sep 21, 2022, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> > It's still not obvious to me why KVM shouldn't do:
> >
> > 	if (!hv_vcpu)
> > 		return false;
> >
> > 	return !(hv_vcpu->cpuid_cache.features_eax & HV_ACCESS_TSC_INVARIANT) ||
> > 	       !(to_kvm_hv(vcpu->kvm)->hv_invtsc_control & HV_INVARIANT_TSC_EXPOSED);
> >
> > I.e. why is invariant TSC _not_ suppressed on Hyper-V by default?
> 
> In case we switch to suppressing invtsc (CPUID.80000007H:EDX[8]) by
> default, i.e. when HV_ACCESS_TSC_INVARIANT was not set in guest visible
> CPUIDs, this is going to be a behavioral change for the already existing
> configurations and we certainly don't want that. It was expirementally
> proven that at least some Windows versions are perfectly happy when they
> see invtsc without this PV feature so I don't see a need to break the
> status quo.
> 
> this is VMM's job, not KVM's. 

Gotcha.  Can you add a comment to capture this?  In particular, the part about it
being KVM's responsibility iff the Hyper-V control is exposed to the guest.

Hmm, and I think it makes to repackage the code so that the "is KVM responsible"
check is separated from the "is the control enabled".  E.g.

	/*
	 * If Hyper-V's invariant TSC control is exposed to the guest, KVM is
	 * responsible for suppressing the invariant TSC CPUID flag if the
	 * Hyper-V control is not enabled.
	 */
	if (!hv_vcpu ||
	    !(hv_vcpu->cpuid_cache.features_eax & HV_ACCESS_TSC_INVARIANT))
		return false;

	return !(to_kvm_hv(vcpu->kvm)->hv_invtsc_control & HV_INVARIANT_TSC_EXPOSED);