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

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Message-ID: <CAHk-=whHZ1KJGVKTaBOSr7KwYAqvrjD9bcoz-SKrsW3DdS9Eug@mail.gmail.com>
Date:   Sun, 3 Sep 2023 13:57:59 -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:
>
> It dodges lockref et al, but it does not dodge SMAP which accounts for
> the difference.

Yeah, just doing that "check if the name is empty" is not free. The
CLAC/STAC overhead is real.

I see no way of avoiding that cost, though - the only true fix is to
fix the glibc braindamage.

In fact, I suspect that my stupid patch is unacceptable exactly
because it actually makes a *real* fstatat() with a real path much
worse due to the "check if the path is empty".

We could possibly move that check into the getname() path, and at
least avoid the extra overhead.

           Linus