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: Mon, 24 Jun 2024 09:36:13 +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 8:07 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sun, 23 Jun 2024 at 13:48, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Now, from having looked a bit at this, I can point you to the
> > differences introduced by having to have the emulation fallback.
>
> Ahh.
>
> Itr does all the same things *and* it has
>
>     "S" (_ptr)
>
> as an added register pressure, because if we take the "call
> cmpxchg8b_emu", we need the address in a fixed place.
>
> I don't see any immediately obvious workaround.

A real fix, not only a workaround, is to rewrite asm arguments to
something like (untested, but "should work"TM):

#define __arch_cmpxchg64_emu(_ptr, _old, _new, _lock_loc, _lock) \
({ \
union __u64_halves o = { .full = (_old), }, \
  n = { .full = (_new), }; \
\
asm volatile(ALTERNATIVE(_lock_loc \
"call cmpxchg8b_emu", \
_lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
    : "+a" (o.low), "+d" (o.high) \
    : [ptr] "S" (_ptr), \
      "b" (n.low), "c" (n.high) \
    : "memory"); \
\
o.full; \
})

(and in a similar way for __arch_try_cmpxchg64_emu).

We already have a memory clobber in the asm template, so to solve the
register pressure issue, just reuse the pointer in %esi to also form
the memory address of cmpxchg8b. GCC is smart enough to do this by
itself, but the current asm template indeed doesn't force "other"
compilers to do so.

Uros.