Re: [PATCH v9 09/16] arm64: errata: Require Apple IMPDEF PMUv3 traps on all CPUs

[Fuad Tabba]

On Mon, 20 Jul 2026 at 06:39, Akihiko Odaki
<odaki@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> KVM relies on ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS as a system-wide
> precondition for PMUv3 emulation on affected Apple CPUs. A local CPU
> erratum is too weak for that: once the workaround is enabled, a late CPU
> that misses it may still be onlined.
>
> Make the workaround an all-early-CPU capability instead. The matcher is
> still local because it depends on MIDR_EL1, but the capability is only
> advertised if all early CPUs match. A late CPU that misses an advertised
> capability is therefore rejected.
>
> Although kept in the errata capability table, this capability describes
> optional trap support rather than a workaround required for safe CPU
> operation. A late CPU may therefore match it when the system does not.
> KVM treats an IMPDEF PMU as unimplemented unless the capability was
> finalized system-wide, so the additional trap support remains unused.
>
> Fixes: bed9b8ec8c71 ("KVM: arm64: Advertise PMUv3 if IMPDEF traps are present")
> Assisted-by: Codex:gpt-5.5
> Signed-off-by: Akihiko Odaki <odaki@xxxxxxxxxxxxxxxxxxxxxx>

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

Cheers,
/fuad



> ---
>  arch/arm64/kernel/cpu_errata.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index 1995e1198648..a5ae628204a1 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -1005,7 +1005,7 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
>         {
>                 .desc = "Apple IMPDEF PMUv3 Traps",
>                 .capability = ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS,
> -               .type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,
> +               .type = ARM64_CPUCAP_EARLY_LOCAL_CPU_FEATURE,
>                 .matches = has_impdef_pmuv3,
>                 .cpu_enable = cpu_enable_impdef_pmuv3_traps,
>         },
>
> --
> 2.55.0
>
>