Re: [PATCH v9 14/16] KVM: arm64: PMU: Implement fixed-counters-only emulation

[Fuad Tabba]

Hi Akihiko,

On Mon, 20 Jul 2026 at 06:39, Akihiko Odaki
<odaki@xxxxxxxxxxxxxxxxxxxxxx> wrote:
...

> +/**
> + * kvm_pmu_create_perf_event - create a perf event for a counter
> + * @pmc: Counter context
> + */
> +static void kvm_pmu_create_perf_event(struct kvm_pmc *pmc)
> +{
> +       struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
> +       struct arm_pmu *arm_pmu = vcpu->kvm->arch.arm_pmu;
> +
> +       if (kvm_pmu_fixed_counters_only(vcpu->kvm)) {
> +               do {
> +                       arm_pmu = kvm_pmu_probe_armpmu(READ_ONCE(vcpu->cpu));
> +
> +                       if (!arm_pmu) {
> +                               pr_warn_once("kvm: Unsupported PMU variation detected.\n");
> +                               add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
> +                               return;
> +                       }

Sashiko flagged that this can leave the perf event out of sync with
the sysreg, which looks plausible: on a pCPU with no registered pPMU
this returns without recreating the event, so a guest event-type
change there updates the sysreg but not the event, and after migrating
back to a pCPU the original pPMU covers, kvm_pmc_enable_perf_event()
re-enables the stale event instead of recreating it.

I know you treat the no-pPMU case as out of spec, but the dropped
update lands once the vCPU is back on a normal pCPU. Is this intended?

Cheers,
/fuad