linux-kernel - Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state

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Message-ID: <20190315100634.GN3567@e103592.cambridge.arm.com>
Date:   Fri, 15 Mar 2019 10:06:37 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Julien Grall <julien.grall@....com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-rt-users@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching
 FPSIMD/SVE state

On Thu, Mar 14, 2019 at 06:07:19PM +0000, Julien Grall wrote:
> Hi Sebastian,
> 
> On 3/4/19 5:25 PM, Sebastian Andrzej Siewior wrote:

[...]

> >It would increase the softirq latency but the question is how bad would
> >it be. It would continue once the SIMD section is done.
> 
> On Arm, the kernel may use either FPSIMD or SVE (if supported by the
> platform). While the FPSIMD context is fairly small (~4K), the SVE context
> can be up to ~64KB.

It's not quite as bad as that.  The FPSIMD state is ~0.5K, with the SVE
state size being up to ~8.5K (though for today's implementations ~2K may
be considered typical).

For comparision, I believe AVX-512 has ~2K of state.

Cheers
---Dave