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

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Message-ID: <CAHk-=wi3p5C1n03UYoQhgVDJbh_0ogCpwbgVGnOdGn6RJ6hnKA@mail.gmail.com>
Date: Fri, 9 Feb 2024 09:55:50 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "Andrew Pinski (QUIC)" <quic_apinski@...cinc.com>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>, Sean Christopherson <seanjc@...gle.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 Fri, 9 Feb 2024 at 09:14, Andrew Pinski (QUIC)
<quic_apinski@...cinc.com> wrote:
>
> So the exact versions of GCC where this is/was fixed are:
> 12.4.0 (not released yet)
> 13.2.0
> 14.1.0 (not released yet)

Looking at the patch that the bugzilla says is the fix, it *looks*
like it's just the "mark volatile" that is missing.

But Sean says that  even if we mark "asm goto" as volatile manually,
it still fails.

So there seems to be something else going on in addition to just the volatile.

Side note: the reason we have that "asm_volatile_goto()" define in the
kernel is that we *used* to have a _different_ workaround for a gcc
bug in this area:

 /*
  * GCC 'asm goto' miscompiles certain code sequences:
  *
  *   http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
  *
  * Work it around via a compiler barrier quirk suggested by Jakub Jelinek.
  *
  * (asm goto is automatically volatile - the naming reflects this.)
  */
 #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)

and looking at that (old) bugzilla there seems to be a lot of "seems
to be fixed", but it's not entirely clear.

We've removed that workaround in commit 43c249ea0b1e ("compiler-gcc.h:
remove ancient workaround for gcc PR 58670"), I'm wondering if maybe
that removal was a bit optimistic.

                Linus