linux-kernel - Re: [RFC PATCH 0/4] x86/percpu: Use segment qualifiers

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Message-ID: <CAHk-=wi4ji10PR35r0FqiKA_XYO38gLbZmPN4SYursP9fiUcXQ@mail.gmail.com>
Date: Tue, 29 Apr 2025 09:49:57 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>, 
	Brian Gerst <brgerst@...il.com>, Denys Vlasenko <dvlasenk@...hat.com>, 
	"H . Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC PATCH 0/4] x86/percpu: Use segment qualifiers

On Tue, 29 Apr 2025 at 09:31, Uros Bizjak <ubizjak@...il.com> wrote:
>
> FYI, after GCC PR 111657 [1] was fixed, gcc-16 will generate the following code:

Well, that's certainly a lot better than the horrible byte-at-a-time loop.

Did you verify doing a structure copy the other way?

Because the segment override on 'rep movs' only works one way - it
only affects the source, not the destination.

So while

      #include <string.h>
      struct a { long arr[30]; };

      __seg_fs struct a m;
      void foo(struct a *dst) { *dst = m; }

works with 'rep movs', changing it do do

      __seg_fs struct a m;
      void foo(struct a *src) { m = *src; }

can only work with a loop of explicit stores.

And I see in the gcc bugzilla that some gcc developer was confused,
and thought this was about calling "memcpy()" (which should complain
about address spaces rather than generate code).

But structure assignment is a different thing.

             Linus