linux-kernel - Re: sve_user

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Message-ID: <20210521091254.GA6675@arm.com>
Date:   Fri, 21 May 2021 10:12:54 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     psodagud@...eaurora.org
Cc:     will@...nel.org, Dave.Martin@....com, amit.kachhap@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Brown <broonie@...nel.org>
Subject: Re: sve_user_discard

On Thu, May 20, 2021 at 04:02:03PM -0700, psodagud@...eaurora.org wrote:
> This is regarding sve_user_disable(CPACR_EL1_ZEN_EL0EN) on every system
> call.  If a userspace task is using SVE instructions and making sys calls in
> between, it would impact the performance of the thread. On every SVE
> instructions after SVC/system call, it would trap to EL1.
> 
> I think by setting CPACR_EL1_ZEN_EL0EN flag,  the processor faults when it
> runs an SVE instruction. This approach may be taken as part of FPSIMD
> registers switching optimizations.  Can below portion of the code use
> thread.fpsimd_cpu and fpsimd_last_state variables to avoid clearing
> CPACR_EL1_ZEN_EL0EN for this kind of use cases?

There were attempts over the past couple of years to optimise the
syscall return use-case. I think the latest is this one:

https://lore.kernel.org/r/20201106193553.22946-2-broonie@kernel.org

I'll let Mark comment on his plans for reviving the series. Do you
happen to have some realistic workload that would be improved by this?
We can always write a micro-benchmark but I wonder how much this matters
in the real world.

-- 
Catalin