On Mon, May 26, 2025, Sandipan Das wrote:
> > @@ -212,6 +212,18 @@ static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
> > bitmap_set(pmu->all_valid_pmc_idx, 0, pmu->nr_arch_gp_counters);
> > }
> >
> > +static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
> > +{
> > + struct vcpu_svm *svm = to_svm(vcpu);
> > +
> > + __amd_pmu_refresh(vcpu);
> > +
> > + if (kvm_rdpmc_in_guest(vcpu))
> > + svm_clr_intercept(svm, INTERCEPT_RDPMC);
> > + else
> > + svm_set_intercept(svm, INTERCEPT_RDPMC);
> > +}
> > +
>
> After putting kprobes on kvm_pmu_rdpmc(), I noticed that RDPMC instructions were
> getting intercepted for the secondary vCPUs. This happens because when secondary
> vCPUs come up, kvm_vcpu_reset() gets called after guest CPUID has been updated.
> While RDPMC interception is initially disabled in the kvm_pmu_refresh() path, it
> gets re-enabled in the kvm_vcpu_reset() path as svm_vcpu_reset() calls init_vmcb().
> We should consider adding the following change to avoid that.
Revisiting this code after the MSR interception rework, I think we should go for
a more complete, big-hammer solution. Rather than manipulate intercepts during
kvm_pmu_refresh(), do the updates as part of the "common" recalc intercepts flow.
And then to trigger recalc on PERF_CAPABILITIES writes, turn KVM_REQ_MSR_FILTER_CHANGED
into a generic KVM_REQ_RECALC_INTERCEPTS.
That way there's one path for calculating dynamic intercepts, which should make it
much more difficult for us to screw up things like reacting to MSR filter changes.
And providing a single path avoids needing to have a series of back-and-forth calls
between common x86 code, PMU code, and vendor code.
