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

[Akihiko Odaki]

On 2026/08/01 1:50, Will Deacon wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> >   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.
It's not only you but Fuad and Sashiko also had the same question:
https://lore.kernel.org/qemu-devel/4cf2911c-610e-4cf1-9c42-cc5792265a89@xxxxxxxxxxxxxxxxxxxxxx/

The answer is that we simply don't enable hypervisor traps. Accessing 
PMUv3 registers will be UNDEFINED on all CPUs, enforcing the 
architectural behavior system-wide.

I plan to change the patch message to clarify this in the next version.

Regards,
Akihiko Odaki