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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 12 Oct 2023 12:40:52 -0700
Message-ID: <CAHk-=whk1WOVzHdoQDm0wM+8ZrpG+zLucWCHhWAcs5OV7M6Q7Q@mail.gmail.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
To:     Nadav Amit <namit@vmware.com>
Cc:     Uros Bizjak <ubizjak@gmail.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nick Desaulniers <ndesaulniers@google.com>
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On Thu, 12 Oct 2023 at 12:32, Nadav Amit <namit@vmware.com> wrote:
>
> If you refer to the difference between DECLARE_PER_CPU_ALIGNED() and
> DECLARE_PER_CPU() - that=E2=80=99s just a silly mistake that I made porti=
ng my
> old patch (I also put =E2=80=9Cconst=E2=80=9D in the wrong place of the d=
eclaration, sorry).

Yes, the only difference is the ALIGNED and the 'const', but I think
also the alias attribute.

However, I'd be happier if we had just one place that declares them, not tw=
o.

Even if the two were identical, it seems wrong to have two
declarations for the same thing.

> The =E2=80=9Ctrick=E2=80=9D that the patch does is to expose a new const_=
pcpu_hot symbol that has
> a =E2=80=9Cconst=E2=80=9D qualifier. For compilation units from which the=
 symbol is effectively
> constant, we use const_pcpu_hot. The compiler then knows that the value w=
ould not
> change.

Oh, I don't disagree with that part.

I just don't see why the 'asm' version would have any difference. For
that too, the compiler should see that the result of the asm doesn't
change.

So my confusion / worry is not about the const alias. I like that part.

My worry is literally "in other situations we _have_ to use asm(), and
it's not clear why gcc wouldn't do as well for it".

                  Linus