linux-kernel - Re: [PATCH 2/2] arm64/sme: Restore SMCR

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Message-ID: <ca380b64-3420-4817-b3b4-584b8640c0ac@sirena.org.uk>
Date: Tue, 30 Jan 2024 14:34:23 +0000
From: Mark Brown <broonie@...nel.org>
To: Dave Martin <Dave.Martin@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Jackson Cooper-Driver <Jackson.Cooper-Driver@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] arm64/sme: Restore SMCR_EL1.EZT0 on exit from suspend

On Tue, Jan 30, 2024 at 10:54:06AM +0000, Dave Martin wrote:
> On Tue, Jan 30, 2024 at 12:02:49AM +0000, Mark Brown wrote:

> > +	if (system_supports_sme2())
> > +		smcr |= SMCR_ELx_EZT0;

> Side question: since ZT0 is likely to be sporadically used, maybe it
> is worth having separate lazy restore for it versus the main SME state?
> (Not relevant for this series though, and probably best deferred until
> there is hardware to benchmark on.  Also, ZT0 is small compared with
> the SME state proper...)

One of the advantages SME has here is that we've got a clear indication
if userspace is actively using the registers through SMSTART and SMSTOP.
We only restore ZT0 at all whenever PSTATE.ZA is set and the strong
recommendation is that should only be set when either ZA or ZT0 are in
active use for power and performance reasons.  While it is likely that
there will be code that uses ZA but doesn't touch ZT0 I would expect
that the overhead of entering the kernel to do a lazy restore will be
sufficiently high for it to be an unreasonable penalty on code that does
touch it, as you say it's not *that* big compared to likely ZA sizes.

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