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

On 2026/07/07 2:04, Oliver Upton wrote:
> 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.

ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS only says that IMPDEF PMUv3 sysregs 
are trapped. It does not say that KVM has a driver-backed PMU usable for 
PMUv3 emulation. This patch checks that extra requirement.

I re-checked CPU hotplug. Onlining a CPU without a PMU later does not 
make an accepted VM unsafe, since the check is against 
cpu_possible_mask. The problem is the reverse case on ACPI: this check 
can disable vPMU when a possible CPU is offline. DT populates 
supported_cpus at boot, while ACPI initially populates it only from 
online CPUs and grows it as matching CPUs come online.

That makes this patch too conservative. In practice, I do not expect 
systems to mix CPUs with and without a usable PMU. A better approach is 
probably to treat such a host as out of spec and add 
TAINT_CPU_OUT_OF_SPEC. We already do that for architectural PMUv3 by 
detecting mismatches in ID_AA64DFR0_EL1.PMUVer; we can do the same for 
ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS with non-standard PMUs.

Regards,
Akihiko Odaki