linux-kernel - Re: [PATCH v1] drivers/perf: apple_m1: fix affinity table for event 0x96 and 0x9b

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Date: Tue, 02 Jul 2024 11:58:00 +0100
From: Marc Zyngier <maz@...nel.org>
To: Yangyu Chen <cyy@...self.name>
Cc: Will Deacon <will@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Mark Rutland <mark.rutland@....com>,
	Janne Grunau <j@...nau.net>,
	Hector Martin <marcan@...can.st>,
	Asahi Lina <lina@...hilina.net>,
	asahi@...ts.linux.dev,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] drivers/perf: apple_m1: fix affinity table for event 0x96 and 0x9b

On Tue, 02 Jul 2024 11:22:21 +0100,
Yangyu Chen <cyy@...self.name> wrote:
> 
> > Yangyu, can you please clarify how you came to the conclusion that
> > these events didn't count anywhere other than counter 7?
> > 
> 
> IIRC, I came across some web page that says events 0x96 and 0x9b
> can only be installed on counter 7 to count Apple AMX, but I can't
> find the page now. Since AMX is not usable in Linux, I don't know
> if this will affect some other instructions that are usable in
> Linux.

As you said, AMX cannot be used with Linux, and that's unlikely to
ever change. But when it comes to the standard ARM ISA, we can only
witness counters 5,6 and 7 being incremented with at the exact same
rate.

So reading between the lines, what I understand is that AMX
instructions would only have their effects counted in counter 7 for
these events, while other instructions would be counted in all 3
counters.

By extension, such behaviour could be applied to SME on HW that
supports it (wild guess).

> There are some other reasons, but I can't say in public.

Fair enough, I'm not asking for the disclosure of anything that isn't
public (the least I know, the better).

> Even though I can't find the actual usage, I think using count 7
> only for these 2 events is safer. If this reason is insufficient,
> we can ignore this patch until we find other evidence that this
> affinity affects some instructions usable in Linux.

I honestly don't mind.

The whole thing is a black box, and is more useful as an interrupt
generator than an actual PMU, due to the lack of freely available
documentation. If the PMU maintainers want to merge this, I won't
oppose it.

Thanks,

	M.

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