Re: [PATCH 1/2] KVM: SVM: Fix TSC_AUX virtualization setup

[Tom Lendacky]

On 9/15/23 12:32, Sean Christopherson wrote:
> On Fri, Sep 15, 2023, Tom Lendacky wrote:
> > On 9/14/23 15:48, Tom Lendacky wrote:
> > > On 9/14/23 15:28, Sean Christopherson wrote:
> > > > On Thu, Sep 14, 2023, Tom Lendacky wrote:
> >
> > > > > +        if (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> > > > > +            svm_clr_intercept(svm, INTERCEPT_RDTSCP);
> > > >
> > > > Same thing here.
> > >
> > > Will do.
> >
> > For RDTSCP, svm_recalc_instruction_intercepts() will set/clear the RDTSCP
> > intercept as part of the svm_vcpu_set_after_cpuid() path, but it will only
> > do it based on kvm_cpu_cap_has(X86_FEATURE_RDTSCP) being true, which is very
> > likely.
> >
> > Do you think that is good enough and we can drop the setting and clearing of
> > the RDTSCP intercept in the sev_es_vcpu_set_after_cpuid() function and only
> > deal with the TSC_AUX MSR intercept?
>
> The common handling should be good enough.
>
> > On a side note, it looks like RDTSCP would not be intercepted if the KVM cap
> > X86_FEATURE_RDTSCP feature is cleared, however unlikely, in
> > kvm_set_cpu_caps() and RDTSCP is not advertised to the guest (assuming the
> > guest is ignoring the RDTSCP CPUID bit).
>
> Hmm, yes, though the only scenario in which KVM clears RDTSCP on AMD comes with
> a WARN (it's a guard against KVM bugs).  If the guest ignores CPUID and uses
> RDTSCP anyways, the guest deserves its death, and leaking the host pCPU doesn't
> seem like a major issue.
>
> That said, if hardware behavior is to ignore unknown intercepts, e.g. if KVM can
> safely set INTERCEPT_RDTSCP even when hardware doesn't support said intercept,
> then I wouldn't be opposed to doing:
>
> 	/*
> 	 * Intercept INVPCID if shadow paging is enabled to sync/free shadow
> 	 * roots, or if INVPCID is disabled in the guest to inject #UD.
> 	 */
> 	if (!kvm_cpu_cap_has(X86_FEATURE_INVPCID) ||
> 	    !npt_enabled || !guest_cpuid_has(&svm->vcpu, X86_FEATURE_INVPCID))
> 		svm_set_intercept(svm, INTERCEPT_INVPCID);
> 	else
> 		svm_clr_intercept(svm, INTERCEPT_INVPCID);
>
> 	if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP) &&
> 	    guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> 		svm_clr_intercept(svm, INTERCEPT_RDTSCP);
> 	else
> 		svm_set_intercept(svm, INTERCEPT_RDTSCP);
>
> Alternatively, KVM could check boot_cpu_has() instead or kvm_cpu_cap_has(), but
> that's not foolproof either, e.g. see Intel's of hiding PCID to workaround the
> TLB flushing bug on Alderlake.  So my vote would either be to keep things as-is,
> or do the above (if that's safe).

Keep things as-is works for me :)

Thanks,
Tom