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

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Date:   Wed, 11 Oct 2023 09:45:36 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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, Oct 11, 2023 at 9:27 AM Uros Bizjak <ubizjak@...il.com> wrote:

> > IOW, something like
> >
> >     static __always_inline unsigned long new_cpu_offset(void)
> >     {
> >         unsigned long res;
> >         asm(ALTERNATIVE(
> >                         "movq " __percpu_arg(1) ",%0",
> >                         "rdgsbase %0",
> >                         X86_FEATURE_FSGSBASE)
> >                 : "=r" (res)
> >                 : "m" (this_cpu_off));
> >         return res;
> >     }
> >
> > would presumably work together with your __seg_gs stuff.
>
> I have zero experience with rdgsbase insn, but the above is not
> dependent on __seg_gs, so (the movq part at least) would also work in
> the current mainline. To work together with __seg_gs stuff,
> this_cpu_offset should be enclosed in __my_cpu_var. Also, if rdgsbase
> is substituted with rdfsbase, it will also work for 32-bit targets.

In fact, rdgsbase is available only for 64-bit targets.

Uros.