linux-kernel - Re: [PATCH v2 4/4] x86/percpu: Use C for percpu read/write accessors

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Date:   Thu, 5 Oct 2023 09:06:17 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Nadav Amit <namit@...are.com>, Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2 4/4] x86/percpu: Use C for percpu read/write accessors


* Uros Bizjak <ubizjak@...il.com> wrote:

> The percpu code mostly uses inline assembly. Using segment qualifiers
> allows to use C code instead, which enables the compiler to perform
> various optimizations (e.g. propagation of memory arguments). Convert
> percpu read and write accessors to C code, so the memory argument can
> be propagated to the instruction that uses this argument.

>    text    data     bss     dec     hex filename
> 25508862        4386540  808388 30703790        1d480ae vmlinux-vanilla.o
> 25500922        4386532  808388 30695842        1d461a2 vmlinux-new.o

Ok, this all looks like a pretty nice optimization.

As discussed previously, I've created a new tip:x86/percpu topic branch
for this, based on tip:x86/asm that carries the other percpu patches.
This branch will be merged in v6.8, best-case scenario.

Also note that I lowered the version cutoff from GCC 13.1 to 12.1, for
the simple selfish reason to include my own daily systems in test coverage.

Is there any known bug fixed in the GCC 12.1 ... 13.1 version range that
could make this approach problematic?

Thanks,

	Ingo