linux-kernel - Re: [PATCH v4] arch/x86: Improve 'rep movs{b|q}' usage in memmove

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Date:   Wed, 16 Mar 2022 17:23:31 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Noah Goldstein <goldstein.w.n@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        hpa@...or.com, luto@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] arch/x86: Improve 'rep movs{b|q}' usage in
 memmove_64.S

On Wed, Nov 17, 2021 at 03:02:45PM -0600, Noah Goldstein wrote:
> Add check for "short distance movsb" for forwards FSRM usage and
> entirely remove backwards 'rep movsq'. Both of these usages hit "slow
> modes" that are an order of magnitude slower than usual.
> 
> 'rep movsb' has some noticeable VERY slow modes that the current
> implementation is either 1) not checking for or 2) intentionally
> using.
> 
> All times are in cycles and measuring the throughput of copying 1024
> bytes.

All these claims need to be proven by

 - real benchmarks - not a microbenchmark - where it shows that
 modifications like that are not "in the noise". Others should be able
 to verify those results too.

 - on a bunch of CPUs from different vendors to verify that they don't
   cause performance regressions on any.

HTH.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette