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

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Message-ID: <CAHk-=wjYOZf2wPj_=arATJ==DQQAQwh0ki=Za0RcE542rWBGFw@mail.gmail.com>
Date:   Sun, 3 Sep 2023 14:05:34 -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 Sun, 3 Sept 2023 at 13:49, Mateusz Guzik <mjguzik@...il.com> wrote:
>
> "real fstat" is syscall(5, fd, &sb).
>
> Sapphire Rapids, will-it-scale, ops/s
>
> stock fstat     5088199
> patched fstat   7625244 (+49%)
> real fstat      8540383 (+67% / +12%)
>
> It dodges lockref et al, but it does not dodge SMAP which accounts for
> the difference.

Side note, since I was looking at this, I hacked up a quick way for
architectures to do their own optimized cp_new_stat() that avoids the
double-buffering.

Sadly it *is* architecture-specific due to padding and
architecture-specific field sizes (and thus EOVERFLOW rules), but it
is what it is.

I don't know how much it matters, but it might make a difference. And
'stat()' is most certainly worth optimizing for, even if glibc has
made our life more difficult.

Want to try out another entirely untested patch? Attached.

                Linus

View attachment "patch.diff" of type "text/x-patch" (3087 bytes)