linux-kernel - Re: arch/x86/include/asm/cmpxchg_32.h:149:9: error: inline assembly requires more registers than available

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Date: Sun, 23 Jun 2024 20:14:29 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: kernel test robot <lkp@...el.com>, oe-kbuild-all@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>, 
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: arch/x86/include/asm/cmpxchg_32.h:149:9: error: inline assembly
 requires more registers than available

On Sun, Jun 23, 2024 at 7:49 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sun, 23 Jun 2024 at 13:43, Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > I disagree with the above.
>
> Your disagreement just doesn't matter.
>
> We don't introduce regressions and then blame others.
>
> There's a very clear rule in kernel development: things that break
> other things ARE NOT FIXES.
>
> EVER.
>
> They get reverted, or the thing they broke gets fixed.
>
> This is not debatable, or something you can "disagree"' with. This is
> how we work, and if you disagree with that, you had better get out of
> kernel development.

I disagree with *I tried to shift the blame to others*! I take full
responsibility for my patch, and I'm perfectly capable of fixing the
breakage with an alternative approach.

I'm OK with the revert, but it won't fix the underlying problem.
Please see the definition of __arch_cmpxchg64_emu - it forces the
address to %esi registers in the same way as __arch_try_cmpxchg64_emu.
Effectively, the compiler allocates 5 input registers just for the
instruction.

The alternative is to implement atomic64_{and,or,xor} as an outline
function, in the same way as e.g. arch_atomic64_add is implemented.
This will avoid the call to __arch_try_cmpxchg64_emu, and the whole
"instruction" will use just:

__alternative_atomic64(add, add_return,
      ASM_OUTPUT2("+A" (i), "+c" (v)),
      ASM_NO_INPUT_CLOBBER("memory"));

> Now, from having looked a bit at this, I can point you to the
> differences introduced by having to have the emulation fallback.

Yes, I know this - I also (runtime!) tested the emulation, but with GCC only.

> And I would personally be ok with leaving Pentium etc entirely behind,
> and getting rid of said fallback, but that's an entirely separate
> thing that will require much more discussion.
>
> Some people still wanted to support old 486 clones not that long ago.

This can be achieved by implementing atomic64_{and,or,xor} as an
outline function.

Uros.