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

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Message-ID: <CAHk-=wiSn8fY4xF-=Bz=qPcbmT_Yzzb4jvvtMBuNQksk94Y=dw@mail.gmail.com>
Date:   Wed, 11 Oct 2023 12:40:09 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Nadav Amit <namit@...are.com>,
        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>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Wed, 11 Oct 2023 at 00:45, Uros Bizjak <ubizjak@...il.com> wrote:
>
> In fact, rdgsbase is available only for 64-bit targets.

Not even all 64-bit targets. That's why I did that ALTERNATIVE() thing
with X86_FEATURE_FSGSBASE, which uses the kernel instruction
re-writing.

So that suggested asm of mine defaults to loading the value from
memory through %gs, but with X86_FEATURE_FSGSBASE it gets rewritten to
use rdgsbase.

And again - I'm not sure it's any faster, but it's _potentially_
certainly better in that it doesn't use the cache-line.

           Linus