Re: [PATCH v9 05/16] KVM: arm64: PMU: Keep implemented counter mask EL-independent

On Mon, 20 Jul 2026 at 06:39, Akihiko Odaki
<[email protected]> wrote:
>
> kvm_pmu_implemented_counter_mask() derives the counter count from
> kvm_vcpu_read_pmcr(). For a nested vCPU outside EL2, PMCR_EL0.N reflects
> MDCR_EL2.HPMN instead of the VM-wide counter count.
>
> Consequently, processing a PMU reload while the saved context is L2 can
> clear and fail to recreate state for counters reserved for EL2.
>
> Build the implemented counter mask from nr_pmu_counters instead. Keep
> the EL-dependent HPMN restriction in
> kvm_pmu_accessible_counter_mask(), where it controls guest access.
>
> Fixes: 600f6fa5c90c ("KVM: arm64: Let kvm_vcpu_read_pmcr() return an EL-dependent value for PMCR_EL0.N")
> Assisted-by: Codex:gpt-5.6-sol
> Signed-off-by: Akihiko Odaki <[email protected]>

Reviewed-by: Fuad Tabba <[email protected]>

Cheers,
/fuad

> ---
>  arch/arm64/kvm/pmu-emul.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index b4b877f31097..5281d37634a0 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -317,12 +317,12 @@ u64 kvm_pmu_accessible_counter_mask(struct kvm_vcpu *vcpu)
>
>  u64 kvm_pmu_implemented_counter_mask(struct kvm_vcpu *vcpu)
>  {
> -       u64 val = FIELD_GET(ARMV8_PMU_PMCR_N, kvm_vcpu_read_pmcr(vcpu));
> +       u64 n = vcpu->kvm->arch.nr_pmu_counters;
>
> -       if (val == 0)
> +       if (n == 0)
>                 return BIT(ARMV8_PMU_CYCLE_IDX);
>         else
> -               return GENMASK(val - 1, 0) | BIT(ARMV8_PMU_CYCLE_IDX);
> +               return GENMASK(n - 1, 0) | BIT(ARMV8_PMU_CYCLE_IDX);
>  }
>
>  static void kvm_pmc_enable_perf_event(struct kvm_pmc *pmc)
>
> --
> 2.55.0
>
>