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

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 09, 2025, Nikunj A. Dadhania wrote:
> On 7/8/2025 8:07 PM, Sean Christopherson wrote:
> > On Mon, Jul 07, 2025, Nikunj A Dadhania wrote:
> >> Introduce the read-only MSR GUEST_TSC_FREQ (0xc0010134) that returns
> >> guest's effective frequency in MHZ when Secure TSC is enabled for SNP
> >> guests. Disable interception of this MSR when Secure TSC is enabled. Note
> >> that GUEST_TSC_FREQ MSR is accessible only to the guest and not from the
> >> hypervisor context.
> > 
> > ...
> > 
> >> @@ -4487,6 +4512,9 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
> >>  
> >>  	/* Can't intercept XSETBV, HV can't modify XCR0 directly */
> >>  	svm_clr_intercept(svm, INTERCEPT_XSETBV);
> >> +
> >> +	if (snp_secure_tsc_enabled(svm->vcpu.kvm))
> >> +		svm_disable_intercept_for_msr(&svm->vcpu, MSR_AMD64_GUEST_TSC_FREQ, MSR_TYPE_RW);
> > 
> > KVM shouldn't be disabling write interception for a read-only MSR. 
> 
> Few of things to consider here:
> 1) GUEST_TSC_FREQ is a *guest only* MSR and what is the point in KVM intercepting writes
>    to that MSR.

Because there's zero point in not intercepting writes, and KVM shouldn't do
things for no reason as doing so tends to confuse readers.  E.g. I reacted to
this because I didn't read the changelog first, and was surprised that the guest
could adjust its TSC frequency (which it obviously can't, but that's what the
code implies to me).

>    The guest vCPU handles it appropriately when interception is disabled.
>
> 2) Guest does not expect GUEST_TSC_FREQ MSR to be intercepted(read or write), guest 
>    will terminate if GUEST_TSC_FREQ MSR is intercepted by the hypervisor:

But it's read-only, the guest shouldn't be writing.  If the vCPU handles #GPs
appropriately, then it should have no problem handling #VCs on bad writes.

> 38cc6495cdec x86/sev: Prevent GUEST_TSC_FREQ MSR interception for Secure TSC enabled guests

That's a guest bug, it shouldn't be complaining about the host intercepting writes.