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 14:25:15 -0400
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Uros Bizjak <ubizjak@...il.com>
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, 23 Jun 2024 at 14:14, Uros Bizjak <ubizjak@...il.com> wrote:
>
> 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.

Ah, good. This is something I'm a bit sensitive about, just because
there's been so many arguments over it over the years, so now I go
into "preemptive nuclear mode" when the regression issue comes up.

Sorry.

> 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.

Oh, I entirely agree that this is a "random compiler implementation"
issue, and then the code around it makes all the difference.

> > 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.

Yeah, crossed emails, I started out just doing the "let's see what the
config difference is", and only after that realized that I had looked
at the wrong code for cmpxchg (ie I had looked at the simpler native
case).

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

Yes, but then a lot of the whole point of commit 95ece48165c1 goes
away, doesn't it?

Or were you suggesting the out-of-line code only for the emulation
case? That would work.

             Linus