Re: [PATCH v9 14/16] KVM: arm64: PMU: Implement fixed-counters-only emulation

[Fuad Tabba]

Hi Akihiko,

On Thu, 30 Jul 2026 at 07:36, Akihiko Odaki
<odaki@xxxxxxxxxxxxxxxxxxxxxx> wrote:
...
> > Sashiko flagged that this can leave the perf event out of sync with
> > the sysreg, which looks plausible: on a pCPU with no registered pPMU
> > this returns without recreating the event, so a guest event-type
> > change there updates the sysreg but not the event, and after migrating
> > back to a pCPU the original pPMU covers, kvm_pmc_enable_perf_event()
> > re-enables the stale event instead of recreating it.
> >
> > I know you treat the no-pPMU case as out of spec, but the dropped
> > update lands once the vCPU is back on a normal pCPU. Is this intended?
>
> "[PATCH v9 06/16] KVM: arm64: PMU: Recreate events after MDCR_EL2
> changes" calls kvm_pmu_stop_counter() in kvm_vcpu_reload_pmu(), so it
> does not matter whether this function calls kvm_pmu_stop_counter().
>
> The other changes that require recreating the event are currently not
> handled at all as Sashiko complains [1]. I'll fix that as done in
> MDCR_EL2 setters in the next version.
>
> That said, I do not intend to add any extra complexity to provide any
> guarantee for a system with unsupported PMU variation except that the
> user can notice and react to that (i.e., the taint is reported and no
> oops happens due to NULL dereference). The patch message is somewhat
> misleading in this aspect so I'm going to fix that in the next version.

I see, yes, fixing the changelog would help.

Thanks,
/fuad

>
> [1]
> https://sashiko.dev/#/patchset/20260720-hybrid-v9-0-2b713ca1b5dc@xxxxxxxxxxxxxxxxxxxxxx?part=6
>
> Regards,
> Akihiko Odaki