Re: [PATCH v8 1/7] KVM: arm64: Disallow vPMU when pPMUs do not cover all CPUs

[Oliver Upton <oupton@kernel.org>]

Hi,

On Mon, Jul 06, 2026 at 07:03:24PM +0900, Akihiko Odaki wrote:
> Commit ec3eb9ed6081 ("KVM: arm64: PMU: Disallow vPMU on non-uniform
> PMUVer") made KVM reject vPMU unless the system-wide PMUVer is usable.
> That covers systems where PMUv3 is absent or non-uniform, as well as
> systems where IMPDEF PMUv3 sysreg traps are unavailable.
> 
> However, KVM can still accept vPMU when all CPUs uniformly trap PMUv3
> sysregs, but the pPMUs registered with KVM only cover a subset of
> possible CPUs.
> 
> Reject vPMU unless the registered pPMUs cover every possible CPU.
> This avoids carrying support for partial pPMU coverage into the
> fixed-counters-only UAPI introduced later in the series.

Doesn't CPU hotplug screw this up? I could online a CPU that doesn't
have a PMU after creating the VM.

I'd rather just change ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS to become a
system feature. That way any CPU which breaks the system-wide assumption
cannot be onlined.

Thanks,
Oliver