linux-kernel - Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu

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Date:   Wed, 18 Oct 2023 11:15:59 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     Nadav Amit <namit@...are.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Wed, 18 Oct 2023 at 11:08, Uros Bizjak <ubizjak@...il.com> wrote:
>
> But loads from non-const memory work like the above.

Yes, I'm certainly ok with the move to use plain loads from __seg_gs
for the percpu accesses. If they didn't honor the memory clobber, we
could never use it at all.

I was just saying that the 'const' alias trick isn't useful for
anything else than 'current', because everything else needs to at
least honor our existing barriers.

(And yes, there's the other user of this_cpu_read_stable() -
'top_of_stack', but as mentioned that doesn't really matter).

          Linus