linux-kernel - Re: [PATCH] Kconfig: Explicitly disable asm goto w/ outputs on gcc-11 (and earlier)

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Date: Thu, 15 Feb 2024 11:17:44 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Jakub Jelinek <jakub@...hat.com>
Cc: Uros Bizjak <ubizjak@...il.com>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Sean Christopherson <seanjc@...gle.com>, "Andrew Pinski (QUIC)" <quic_apinski@...cinc.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Masahiro Yamada <masahiroy@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH] Kconfig: Explicitly disable asm goto w/ outputs on gcc-11
 (and earlier)

On Thu, 15 Feb 2024 at 10:25, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, 15 Feb 2024 at 00:39, Jakub Jelinek <jakub@...hat.com> wrote:
> >
> > Can it be guarded with
> > #if GCC_VERSION < 140100
>
> Ack. I'll update the workaround to do that, and add the new and
> improved bugzilla pointer.

. and I also followed your suggestion to just consider any gcc-14
snapshots as fixed. That seemed safe, considering that in practice the
actual bug that Sean reported seems to not actually trigger with any
gcc version 12.1+ as per your bisect (and my minimal testing).

HOWEVER, when I was working through this, I noted that the *other*
part of the workaround (the "missing volatile") doesn't seem to have
been backported as aggressively.

IOW, I find that "Mark asm goto with outputs as volatile" in the
gcc-12 and gcc-13 branches, but not in gcc-11.

So I did end up making the default "asm_goto_output()" macro always
use "asm volatile goto()", so that we don't have to worry about the
other gcc issue.

End result: the extra empty asm barrier is now dependent on gcc
version in my tree, but the manual addition of 'volatile' is
unconditional.

Because it looks to me like gcc-11.5 will have your fix for the pseudo
ordering, but not Andrew Pinski's fix for missing a volatile.

Anyway, since the version dependencies were complex enough, I ended up
going with putting that logic in our Kconfig files, rather than making
the gcc-specific header file an ugly mess of #if's.

Our Kconfig files are pretty much designed for having complicated
configuration dependencies, so it ends up being quite natural there:

  config GCC_ASM_GOTO_OUTPUT_WORKAROUND
        bool
        depends on CC_IS_GCC && CC_HAS_ASM_GOTO_OUTPUT
        # Fixed in GCC 14.1, 13.3, 12.4 and 11.5
        # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921
        default y if GCC_VERSION < 110500
        default y if GCC_VERSION >= 120000 && GCC_VERSION < 120400
        default y if GCC_VERSION >= 130000 && GCC_VERSION < 130300

and having those kinds of horrid expressions as preprocessor code
included in every single compilation unit seemed just nasty.

              Linus