Re: [PATCH v8 2/2] KVM: SVM: Enable Secure TSC for SNP guests

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 08, 2025, Kai Huang wrote:
> > > Even some bug results in the default_tsc_khz being 0, will the
> > > SNP_LAUNCH_START command catch this and return error?
> > 
> > No, that is an invalid configuration, desired_tsc_khz is set to 0 when
> > SecureTSC is disabled. If SecureTSC is enabled, desired_tsc_khz should
> > have correct value.
> 
> So it's an invalid configuration that when Secure TSC is enabled and
> desired_tsc_khz is 0.  Assuming the SNP_LAUNCH_START will return an error
> if such configuration is used, wouldn't it be simpler if you remove the
> above check and depend on the SNP_LAUNCH_START command to catch the
> invalid configuration?

Support for secure TSC should depend on tsc_khz being non-zero.  That way it'll
be impossible for arch.default_tsc_khz to be zero at runtime.  Then KVM can WARN
on arch.default_tsc_khz being zero during SNP_LAUNCH_START.

I.e.

	if (sev_snp_enabled && tsc_khz &&
	    cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
		sev_supported_vmsa_features |= SVM_SEV_FEAT_SECURE_TSC;