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

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Date:   Thu, 19 Oct 2023 11:07:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Uros Bizjak <ubizjak@...il.com>, 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>,
        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, Oct 18, 2023 at 03:40:05PM -0700, Linus Torvalds wrote:
> On Wed, 18 Oct 2023 at 14:40, Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > The ones in "raw" form are not IRQ safe and these are implemented
> > without volatile qualifier.
> 
> You are misreading it.
> 
> Both *are* irq safe - on x86.

Stronger, x86 arch code very much relies on them being NMI-safe. Which
makes the generic implementation insufficient. 

They *must* be single RmW instructions on x86.