linux-kernel - Re: [PATCH 1/3] arm64/fpsimd: Ensure SME storage is allocated after SVE VL changes

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Message-ID: <3ce4fc14-bd43-47e9-aa37-59bb3cd5d051@sirena.org.uk>
Date:   Thu, 20 Jul 2023 13:27:10 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Shuah Khan <shuah@...nel.org>,
        David Spickett <David.Spickett@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] arm64/fpsimd: Ensure SME storage is allocated after
 SVE VL changes

On Thu, Jul 20, 2023 at 11:52:36AM +0100, Will Deacon wrote:
> On Thu, Jul 13, 2023 at 09:06:04PM +0100, Mark Brown wrote:

> > Since the ABI does not specify that changing the SVE vector length disturbs
> > SME state, and since SVE code may not be aware of SME code in the process,
> > we shouldn't simply discard any ZA state. Instead immediately reallocate
> > the storage for SVE if SME is active, and disable SME if we change the SVE
> > vector length while there is no SME state active.

> What is the advantage of keep the old behaviour in this case? In other
> words, if it's acceptable to reallocate the state when SME is active, why
> not just reallocate in all cases?

It was minimising the changes to the status quo given how attached
people often are to these things.

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