linux-kernel - Re: [RFC PATCH 0/5] running kernel mode SIMD with softirqs disabled

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Date:   Thu, 14 Jan 2021 09:22:49 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Martin <dave.martin@....com>,
        Mark Brown <broonie@...nel.org>,
        Eric Biggers <ebiggers@...nel.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH 0/5] running kernel mode SIMD with softirqs disabled

On Sat, 19 Dec 2020 at 03:05, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Fri, Dec 18, 2020 at 06:01:01PM +0100, Ard Biesheuvel wrote:
> >
> > Questions:
> > - what did I miss or break horribly?
> > - does any of this matter for RT? AIUI, RT runs softirqs from a dedicated
> >   kthread, so I don't think it cares.
> > - what would be a reasonable upper bound to keep softirqs disabled? I suppose
> >   100s of cycles or less is overkill, but I'm not sure how to derive a better
> >   answer.
> > - could we do the same on x86, now that kernel_fpu_begin/end is no longer
> >   expensive?
>
> If this approach works not only would it allow us to support the
> synchronous users better, it would also allow us to remove loads
> of cruft in the Crypto API that exist solely to support these SIMD
> code paths.
>
> So I eagerly await the assessment of the scheduler/RT folks on this
> approach.
>

Any insights here? Is there a ballpark upper bound for the duration of
a softirq disabled section? Other reasons why dis/enabling softirq
handling is a bad idea?