linux-kernel - Re: [PATCH 00/18] KVM: arm64: Support FEAT

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Message-ID: <Z38E4hRMlBp3CBz0@linux.dev>
Date: Wed, 8 Jan 2025 15:06:10 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: Marc Zyngier <maz@...nel.org>
Cc: Will Deacon <will@...nel.org>, kvmarm@...ts.linux.dev,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Mingwei Zhang <mizhang@...gle.com>,
	Colton Lewis <coltonlewis@...gle.com>,
	Raghavendra Rao Ananta <rananta@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Mark Rutland <mark.rutland@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/18] KVM: arm64: Support FEAT_PMUv3 on Apple hardware\

On Wed, Jan 08, 2025 at 09:26:54PM +0000, Marc Zyngier wrote:
> On Wed, 08 Jan 2025 20:14:07 +0000,
> Oliver Upton <oliver.upton@...ux.dev> wrote:
> > 
> > Hey Will,
> > 
> > On Wed, Jan 08, 2025 at 12:38:41PM +0000, Will Deacon wrote:
> > > What's your plan for this series? I started looking at it and I can take
> > > the first four apple_m1 patches if you like?
> > 
> > I plan on posting a respin of it by next week, which should look pretty
> > much the same besides cleaning up the build error I introduced :)
> > 
> > Besides that, I think we need to decide on the KVM side of things
> > whether or not we want to support an event counter in addition to the
> > PMU cycle counter. Janne's FEX use case would certainly benefit from it.
> 
> I think we should always be able to support *one* counter on top of
> the cycle counter. Doing more than that would result in inconsistent
> behaviours (some events only count on a single counter).
> 
> Unless we restrict ourselves to a very small set of events that we can
> always schedule on any counter, but this doesn't sound very promising.

I definitely agree that a single event counter is the way to go. Dealing
with this IMPDEF crud is gross already, and coping with event affinities
would only make it worse.

I was more wanting to test the idea that we want programmable event
counters at all, although it isn't that much of a burden on top of the
cycle counter.

I'll un-RFC the tail of the series in v2 then.

-- 
Thanks,
Oliver