linux-kernel - Re: [PATCH v2] x86: bring back rep movsq for user access on CPUs without ERMS

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Message-ID: <CAHk-=wh5s3JbYKd4SsCsDQ8RxCEKXGG-d61Ab9hOev9wnyGbHg@mail.gmail.com>
Date:   Wed, 30 Aug 2023 09:50:46 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        bp@...en8.de
Subject: Re: [PATCH v2] x86: bring back rep movsq for user access on CPUs
 without ERMS

On Wed, 30 Aug 2023 at 07:03, Mateusz Guzik <mjguzik@...il.com> wrote:
>
> Hand-rolled mov loops executing in this case are quite pessimal compared
> to rep movsq for bigger sizes. While the upper limit depends on uarch,
> everyone is well south of 1KB AFAICS and sizes bigger than that are
> common.
>
> While technically ancient CPUs may be suffering from rep usage, gcc has
> been emitting it for years all over kernel code, so I don't think this
> is a legitimate concern.
>
> Sample result from read1_processes from will-it-scale (4KB reads/s):
> before: 1507021
> after:  1721828 (+14%)

Ok, patch looks fine to me now.

So I applied this directly to my tree, since I was the one doing the
x86 memcpy cleanups that removed the REP_GOOD hackery anyway.

              Linus