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

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Message-ID: <20190214103449.GN3567@e103592.cambridge.arm.com>
Date:   Thu, 14 Feb 2019 10:34:52 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Julien Grall <julien.grall@....com>,
        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 Wed, Feb 13, 2019 at 05:52:27PM +0100, Sebastian Andrzej Siewior wrote:
> On 2019-02-13 16:40:00 [+0100], Ard Biesheuvel wrote:
> > > > This is equal what x86 is currently doing. The naming is slightly
> > > > different, there is irq_fpu_usable().
> > >
> > > Yes, I think it's basically the same idea.
> > >
> > > It's been evolving a bit on both sides, but is quite similar now.
> > >
> > 
> > may_use_simd() only exists because we have a generic crypto SIMD
> > helper, and so we needed something arch agnostic to wrap around
> > irq_fpu_usable()
> 
> My question was more if this is helpful and we want to keep or if
> it would be better to remove it and always disable BH as part of SIMD
> operations.

Wouldn't this arbitrarily increase softirq latency?  Unconditionally
forbidding SIMD in softirq might make more sense.  It depends on how
important the use cases are...

Cheers
---Dave