Re: [PATCH RFC] KVM: arm64: PMU: Use multiple host PMUs

[Marc Zyngier <maz@kernel.org>]

On Wed, 19 Mar 2025 11:26:18 +0000,
Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> 
> On 2025/03/19 20:07, Marc Zyngier wrote:
> > On Wed, 19 Mar 2025 10:26:57 +0000,
> >>
> > But that'd be a new ABI, which again would require buy-in from
> > userspace.  Maybe there is scope for an all CPUs, cycle-counter only
> > PMUv3 exposed to the guest, but that cannot be set automatically, as
> > we would otherwise regress existing setups.
> > 
> > At this stage, and given that you need to change userspace, I'm not
> > sure what the best course of action is.
> 
> Having an explicit flag for the userspace is fine for QEMU, which I
> care. It can flip the flag if and only if threads are not pinned to
> one PMU and the machine is a new setup.
> 
> I also wonder what regression you think setting it automatically causes.

The current behaviour is that if you don't specify anything other than
creating a PMUv3 (without KVM_ARM_VCPU_PMU_V3_SET_PMU), you get *some*
PMU, and userspace is responsible for running the vcpu on CPUs that
will implement that PMU. When if does, all the counters, all the
events are valid. If it doesn't, nothing counts, but the
counters/events are still valid.

If you now add this flag automatically, the guest doesn't see the full
PMU anymore. Only the cycle counter. That's the regression.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.