[PATCH v2 0/4] KVM: arm64: Improve PMU support on heterogeneous systems

[Reiji Watanabe]

On Tue, Dec 14, 2021 at 3:57 AM Marc Zyngier <maz at kernel.org> wrote:
>
> On Tue, 14 Dec 2021 06:24:38 +0000,
> Reiji Watanabe <reijiw at google.com> wrote:
> >
> > Hi Alex,
> >
> > On Mon, Dec 13, 2021 at 3:14 AM Alexandru Elisei
> > <alexandru.elisei at arm.com> wrote:
> > >
> > > Also, as VCPUs get migrated from one physical CPU to the other, the
> > > semantics of the microarchitectural events change, even if the event ID is
> > > the same.
> >
> > Yes, I understand.  As mentioned, this can work only when the
> > CPU affinity is set for vCPU threads appropriately (, which could
> > be done even without changing userspace).
>
> Implicit bindings to random PMUs based on the scheduling seems a
> pretty fragile API to me,

Yes, I understand that. I was just looking into the possibility
of improving the default behavior in some way rather than keeping
the unpredictable default behavior.

> and presents no real incentive for userspace
> to start doing the right thing.

I see... It makes sense.
I didn't think about that aspect.

> I'd prefer not counting events at all when on the wrong CPU (for some
> definition of 'wrong'), rather than accumulating unrelated events.
> Both are admittedly wrong, but between two evils, I'd rather stick
> with the one I know (and that doesn't require any change)...
>
> Alex's series brings a way to solve this by allowing userspace to pick
> a PMU and make sure userspace is aware of the consequences. It puts
> userspace in charge, and doesn't leave space for ambiguous behaviours.
>
> I definitely find value in this approach.

Yes, I agree with that.
It wasn't meant to replace Alex's approach.  It was only about the
default behavior (i.e. when userspace does not specify a PMUID with
the new API).

Anyway, thank you so much for sharing your thoughts on it !

Regards,
Reiji