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

[Sean Christopherson <seanjc@google.com>]

On Mon, Apr 27, 2026, Yosry Ahmed wrote:
> > static inline void __kvm_pmu_reprogram_counters(struct kvm_pmu *pmu, u64 diff,
> >                                                 bool defer)
> > {
> >         struct kvm_vcpu *vcpu = pmu_to_vcpu(pmu);
> >
> >         lockdep_assert_once(defer || kvm_get_running_vcpu() == vcpu);
> >
> >         if (!diff)
> >                 return;
> >
> >         atomic64_or(diff, &pmu->__reprogram_pmi);
> >
> >         if (defer)
> >                 kvm_make_request(KVM_REQ_PMU, vcpu);
> >         else
> >                 kvm_pmu_handle_event(pmu_to_vcpu(pmu));
> > }
> 
> I like that the KVM PMU code is now presenting a generic API to
> reprogram counters rather than handling nested transitions, even
> though reprogram_on_nested_transition fits better semantically in
> kvm_pmu (than svm_nested_state).
> 
> I do have a few questions:
> 
> 1. Do we want to do all of the work in kvm_pmu_handle_event() on every
> nested transition (rather than just reprogram counters)? Genuinely
> asking as I am not sure if the rest of it is significant.

Yes, we have to for correctness.  And somewhat sneakily, it's not that as much
work as it might seem at first glance because the Host/Guest stuff is limited to
the mediated PMU.  Specifically, pmu->need_cleanup will never be true and so the
heavy-ish kvm_pmu_cleanup() will never be invoked.

As for correctness, we either need to run through this code:

	kvm_for_each_pmc(pmu, pmc, bit, bitmap)
		kvm_pmu_recalc_pmc_emulation(pmu, pmc);

or pend a KVM_REQ_PMU so that it's done before re-entering the guest, so that
KVM does the right thing when skipping/emulating guest instructions.  That flow
is relatively cheap, so I don't see any reason to defer it.


> 2. This approach will reprogram all counters that need it on nested
> transitions. In my proposed approach above, I only iterate over
> counters in reprogram_on_nested_transition and reprogram them. Do you
> think it matters? I guess if other counters need reprogramming we'll
> probably do it in kvm_pmu_handle_event() before running the vCPU
> anyway,

Correct.  KVM has to do the work before the next VMRUN, all we're doing is
completing the work earlier than is strictly necessary.

> but then we're repeating the work here?

No, it's not repeated.  That's why I want to callkvm_pmu_handle_event(): it
updates pmu->reprogram_pmi to clear bits for PMCs that are successfully reprogrammed.

> 3. In this world we still keep the mediated_reprogram_counter() callback, right?

Weren't we planning on a callback that would take the diff of counters?  I.e. one
callback per kvm_pmu_handle_event(), not one callback per PMC?