Re: [PATCH v9 03/16] KVM: arm64: PMU: Freeze counter count after first run

[Fuad Tabba]

On Mon, 20 Jul 2026 at 06:39, Akihiko Odaki
<odaki@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> PMU configuration is VM-scoped, but SET_NR_COUNTERS can be issued
> through any vCPU. Checking only whether that vCPU's PMU has been
> initialized allows userspace to change the counter count through an idle
> sibling after another vCPU has run.
>
> Reject the attribute once any vCPU has run. This keeps the VM-wide
> implemented counter mask and the PMCR_EL0.N value stable after guest
> execution begins.
>
> Fixes: b7628c797376 ("KVM: arm64: Allow userspace to limit the number of PMU counters for EL2 VMs")
> Assisted-by: Codex:gpt-5.6-sol
> Signed-off-by: Akihiko Odaki <odaki@xxxxxxxxxxxxxxxxxxxxxx>

Reviewed-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>

Cheers,
/fuad

> ---
>  arch/arm64/kvm/pmu-emul.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index b5df6843dbcd..b4b877f31097 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -1087,6 +1087,9 @@ static int kvm_arm_pmu_v3_set_nr_counters(struct kvm_vcpu *vcpu, unsigned int n)
>  {
>         struct kvm *kvm = vcpu->kvm;
>
> +       if (kvm_vm_has_ran_once(kvm))
> +               return -EBUSY;
> +
>         if (!kvm->arch.arm_pmu)
>                 return -EINVAL;
>
>
> --
> 2.55.0
>
>