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

[Marc Zyngier]

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

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.

	M.

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