Re: [PATCH v8 04/11] arm64: errata: Require Apple IMPDEF PMUv3 traps on all CPUs

[Will Deacon <will@kernel.org>]

On Fri, Jul 10, 2026 at 08:14:58PM +0900, Akihiko Odaki 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, and late CPUs that miss it are
> rejected once the capability is enabled.
> 
> Assisted-by: Codex:gpt-5.5
> Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> ---
>  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 476a37c82108..14ce49d693c4 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -964,7 +964,7 @@ 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,

I always get tangled in knots with this code, but does this correctly
handle the case where the boot/early CPUs don't have the impdef PMU
feature but late-onlined CPUs do? I'm not sure we configure the traps
correctly in that case but this is all gloriously undocumented.

Will