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

[Akihiko Odaki]

On 2025/03/20 18:10, Marc Zyngier wrote:
> On Thu, 20 Mar 2025 06:03:35 +0000,
> Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> > On 2025/03/20 3:51, Oliver Upton wrote:
> > > On Wed, Mar 19, 2025 at 06:38:38PM +0000, Marc Zyngier wrote:
> > > > On Wed, 19 Mar 2025 11:51:21 +0000, Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> > > > > What about setting the flag automatically when a user fails to pin
> > > > > vCPUs to CPUs that are covered by one PMU? There would be no change if
> > > > > a user correctly pins vCPUs as it is. Otherwise, they will see a
> > > > > correct feature set advertised to the guest and the cycle counter
> > > > > working.
> > > >
> > > > How do you know that the affinity is "correct"? VCPU affinity can be
> > > > changed at any time. I, for one, do not want my VMs to change
> > > > behaviour because I let the vcpus bounce around as the scheduler sees
> > > > fit.
> >
> > Checking the affinity when picking the default PMU; the vCPU affinity
> > is the only thing that rules the choice of the default PMU even now.
> >
> > Perhaps we may model the API as follows: introduce another "composite"
> > PMU that works on any core but only exposes the cycle counter. Robust
> > VMMs will choose it or one of hardware PMUs with
> > KVM_ARM_VCPU_PMU_V3_SET_PMU. KVM will choose the default PMU according
> > to the vCPU affinity at the point of KVM_ARM_VCPU_INIT otherwise. If
> > the affinity is covered by one hardware PMU, that PMU will be chosen
> > as the default. The "composite" PMU will be the default otherwise.
>
> This makes no sense to me. A VCPU is always affine to a PMU, because
> we do not support configurations where only some CPUs have a PMU. This
> is an all-or-nothing situation.

At least isn't it fine to have the composite PMU with a new value for 
KVM_ARM_VCPU_PMU_V3_SET_PMU?

> More importantly, you keep suggesting the same "new default", and I
> keep saying NO.
>
> My position is clear: if you want a *new* behaviour, you *must* add a
> new flag that the VMM explicitly provides to enable this CC-only PMU.
> No change in default behaviour at all.
>
> I'm not going to move from that.

Why not? It will not break anything guaranteed to work in the past.

Currently KVM only guarantees that the emulated PMU correctly counts 
only when
1) the vCPU affinity is contained by one PMU and
2) it will not expand

Breaking these conditions will make the behavior of the emulated PMU 
undefined. Now I'm proposing to remove 1).

Regards,
Akihiko Odaki

> 	M.