Re: [PATCH v7] target/arm: Always add pmu property for host and max

On 2026/07/28 1:10, Peter Maydell wrote:
> On Mon, 29 Jun 2026 at 09:10, Akihiko Odaki
> <[email protected]> wrote:
>>
>> kvm-steal-time and sve properties are added for KVM even if the
>> corresponding features are not available. Always add pmu property for
>> "host" and "max".
>>
>> Note that we still don't add the property for other CPUs that lack PMU.
>> This is because we do not know what a PMU version should be enabled
>> when the user sets the property to true while it is defined as an error
>> for the "host" or "max" CPU when the hardware accelerator is enabled and
>> the host doesn't have a PMU.
>>
>> This fixes qtest-aarch64/arm-cpu-features on the hosts that supports
>> KVM but doesn't support PMU emulation.
> 
> No, we should not special case "host" and "max" like this.
> The "pmu" property exists only if the CPU has a PMU and it can
> be turned off. If The arm-cpu-features test assumes that the
> host CPU always has a PMU and so always has the property, then
> that's a bug in the test, not in the code handling the property.

The test correctly avoids assuming that the host always has a PMU. It 
expects that the run-time-derived "host" CPU has a stable "pmu" 
property, with its value reporting whether PMU is available.

That is already how Arm handles SVE. Commit d25293068b03 ("target/arm: 
Move kvm test out of cpu_arm_set_sve") added a false SVE property when 
KVM lacks SVE. PAuth and kvm-steal-time use the same general model. 
Other architectures do the similar; for example LoongArch KVM does 
exactly this for "pmu". So we are avoiding special-casing the Arm pmu 
property here.

The comment in aarch64_add_sve_properties() gives the rationale 
explicitly: "to allow -cpu max,sve=off to always be valid." sve=on still 
fails in that case. Keeping the property stable prevents -cpu 
host,pmu=off from becoming invalid on hosts where the PMU cannot be 
enabled. This also matters outside the test: libvirt emits pmu=off when 
the PMU is explicitly disabled. This design is intended to follow the 
alternative you described previously [1].

"host" and "max" are special-cased for a different reason. For fixed Arm 
CPU models, the property is omitted because its semantics is not defined 
when they don't have PMUs. "host" and "max" are different: "host" and 
"max" enable whatever PMU available, and fail otherwise.

[1] 
https://lore.kernel.org/qemu-devel/CAFEAcA_HWfCU09NfZDf6EC=rpvHn148avySCztQ8PqPBMFx4_Q@mail.gmail.com/

Regards,
Akihiko Odaki