Re: [PATCH v4 3/6] KVM: x86/pmu: Disable counters based on Host-Only/Guest-Only bits in SVM

Yosry Ahmed
2026-04-24T06:55:53Z

On Mon, Apr 06, 2026 at 06:30:05PM -0700, Sean Christopherson wrote:
> On Thu, Mar 26, 2026, Yosry Ahmed wrote:
> > diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
> > index ff5acb8b199b0..5961c002b28eb 100644
> > --- a/arch/x86/include/asm/perf_event.h
> > +++ b/arch/x86/include/asm/perf_event.h
> > @@ -60,6 +60,8 @@
> > #define AMD64_EVENTSEL_INT_CORE_ENABLE (1ULL << 36)
> > #define AMD64_EVENTSEL_GUESTONLY (1ULL << 40)
> > #define AMD64_EVENTSEL_HOSTONLY (1ULL << 41)
> > +#define AMD64_EVENTSEL_HOST_GUEST_MASK \
> > + (AMD64_EVENTSEL_HOSTONLY | AMD64_EVENTSEL_GUESTONLY)
> >
> > #define AMD64_EVENTSEL_INT_CORE_SEL_SHIFT 37
> > #define AMD64_EVENTSEL_INT_CORE_SEL_MASK \
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index d6ac3c55fce55..e35d598f809a2 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -559,6 +559,7 @@ static int reprogram_counter(struct kvm_pmc *pmc)
> >
> > if (kvm_vcpu_has_mediated_pmu(pmu_to_vcpu(pmu))) {
> > kvm_mediated_pmu_refresh_event_filter(pmc);
> > + kvm_pmu_call(mediated_reprogram_counter)(pmc);
>
> I would rather make a single call from kvm_pmu_handle_event(), and let the vendor
> deal with mediated vs. legacy. I want to avoid mediated-specific ops as much as
> possible, and I think kvm_x86_ops.reprogram_counters() would be easier to
> understand overall.

I think this doesn't apply anymore now that most nested transitions
won't be handled through kvm_pmu_handle_event(). Also because we need
kvm_mediated_pmu_refresh_event_filter() to still be called before
re-evaluating H/G bits and EFER.SVME.

I think leave this callback as-is and handle everything through
reprogram_counter(). Export reprogram_counter() and rename it to
kvm_pmu_reprogram_counter(), and end up with something like this:

void __kvm_pmu_handle_nested_transition(struct kvm_vcpu *vcpu, bool defer)
{
struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
DECLARE_BITMAP(bitmap, X86_PMC_IDX_MAX);

if (bitmap_empty(pmu->reprogram_on_nested_transition, X86_PMC_IDX_MAX))
return;

bitmap_copy(bitmap, pmu->reprogram_pmi, X86_PMC_IDX_MAX);
bitmap_zero(pmu->reprogram_on_nested_transition, X86_PMC_IDX_MAX);

BUILD_BUG_ON(sizeof(pmu->reprogram_on_nested_transition) != sizeof(atomic64_t));
if (defer) {
atomic64_or(*(s64 *)pmu->reprogram_on_nested_transition,
&vcpu_to_pmu(vcpu)->__reprogram_pmi);
kvm_make_request(KVM_REQ_PMU, vcpu);
return;
}

kvm_for_each_pmc(pmu, pmc, bit, bitmap)
kvm_pmu_reprogram_counter(pmc);
}

void kvm_pmu_handle_nested_transition(struct kvm_vcpu *vcpu)
{
__kvm_pmu_handle_nested_transition(vcpu, false);
}

Actually, if that's desired, we can move this logic into SVM code now.
We won't be calling kvm_pmu_handle_nested_transition() from inside
enter_guest_mode() and leave_guest_mode() anyway so that we can only
defer for the svm_leave_nested() path.

So we can move:
- kvm_pmu_handle_nested_transition() to
svm_pmu_handle_nested_transition()
- pmu->reprogram_on_nested_transition to
svm->nested.reprogram_on_nested_transition

Not sure if we want to keep SVM-specific logic in SVM code, or if we
want to keep code generic as much as possible. I can see good arguments
for both stances.

> > +
> > +static void amd_mediated_pmu_reprogram_counter(struct kvm_pmc *pmc)
> > +{
> > + amd_mediated_pmu_handle_host_guest_bits(pmc);
>
> And then this doesn't need to be such a wonky wrapper, and the "reprogram on
> nested transition" logic can also clear the entire bitmap instead of doing things
> piecemeal, e.g. it can be something like so in the end:

We can still clear the entire bitmap in one go with the above
suggestion, but we'll keep the wonky wrapper :)

>
> if (!kvm_vcpu_has_mediated_pmu(vcpu))
> return;
>
> bitmap_zero(pmu->pmc_reprogram_on_nested_transition, X86_PMC_IDX_MAX);
>
> kvm_for_each_pmc(pmu, pmc, bit, bitmap)
> amd_mediated_pmu_handle_host_guest_bits(pmc);