Re: [PATCH v8 09/11] KVM: arm64: PMU: Implement fixed-counters-only emulation

[Akihiko Odaki]

On 2026/07/13 16:41, Oliver Upton wrote:
> On Fri, Jul 10, 2026 at 08:15:03PM +0900, Akihiko Odaki wrote:
> > @@ -813,6 +842,15 @@ void kvm_host_pmu_init(struct arm_pmu *pmu)
> >   	if (!pmuv3_implemented(kvm_arm_pmu_get_pmuver_limit()))
> >   		return;
> >   
> > +	/*
> > +	 * IMPDEF PMUv3 traps are non-architectural, and KVM cannot assume a
> > +	 * uniform PMUv3-compatible arm_pmu is available on all CPUs.
> > +	 */
> > +	if (cpus_have_final_cap(ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS)) {
> > +		kvm_info("Non-architectural PMU, tainting kernel\n");
> > +		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
> > +	}
> > +
>
> This is an unrelated change, and really the taint should be added with the
> .cpu_enable() for this capability. That's the point where we flip the
> magic bit.

I'm abandoning this change because it does not effectively cover all 
corner cases we don't want to support:
https://lore.kernel.org/all/0e5e64ea-2f3c-4ce6-88ac-08b66f13027c@xxxxxxxxxxxxxxxxxxxxxx/

> > +void kvm_vcpu_load_pmu(struct kvm_vcpu *vcpu, int last_cpu)
> > +{
> > +	if (!kvm_pmu_fixed_counters_only(vcpu->kvm) || vcpu->cpu == last_cpu || last_cpu == -1)
> 										^~~~~~~~~~~~~~
>
> Does this do anything other than avoid a spurious reload on the first KVM_RUN?

Yes. Without this check, the value will be passed to cpu_max_bits_warn() 
in the following call chain, resulting in a warning ifdef 
CONFIG_DEBUG_PER_CPU_MAPS:

1. kvm_vcpu_load_pmu()
2. kvm_pmu_probe_armpmu()
3. cpumask_test_cpu()
4. cpumask_check()
5. cpu_max_bits_warn()

Regards,
Akihiko Odaki