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

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Date:   Sun, 3 Sep 2023 22:48:58 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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, Sep 03, 2023 at 01:08:03PM -0700, Linus Torvalds wrote:
> On Sun, 3 Sept 2023 at 11:49, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > So I have no idea why you claim that "currently they have no choice".
> > glibc is simply being incredibly stupid, and using newfstatat() for no
> > good reason.
> 
> Do you have any good benchmark that shows the effects of this?
> 
> And if you do, does the attached patch (ENTIRELY UNTESTED!) fix the
> silly glibc mis-feature?
> 

"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.