linux-kernel - Re: [PATCH 3/3] locking/atomic/x86: Introduce arch_try_cmpxchg64() for !CONFIG_X86

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Date: Tue, 9 Apr 2024 09:50:50 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/3] locking/atomic/x86: Introduce arch_try_cmpxchg64()
 for !CONFIG_X86_CMPXCHG64


* Uros Bizjak <ubizjak@...il.com> wrote:

> Commit:
> 
>   6d12c8d308e68 ("percpu: Wire up cmpxchg128")
> 
> improved emulated cmpxchg8b_emu() library function to return
> success/failure in a ZF flag.
> 
> Define arch_try_cmpxchg64() for !CONFIG_X86_CMPXCHG64 targets
> to override the generic archy_try_cmpxchg() with an optimized
> target specific implementation that handles ZF flag.
> 
> The assembly code at the call sites improves from:
> 
>    bf56d:	e8 fc ff ff ff       	call   cmpxchg8b_emu
>    bf572:	8b 74 24 28          	mov    0x28(%esp),%esi
>    bf576:	89 c3                	mov    %eax,%ebx
>    bf578:	89 d1                	mov    %edx,%ecx
>    bf57a:	8b 7c 24 2c          	mov    0x2c(%esp),%edi
>    bf57e:	89 f0                	mov    %esi,%eax
>    bf580:	89 fa                	mov    %edi,%edx
>    bf582:	31 d8                	xor    %ebx,%eax
>    bf584:	31 ca                	xor    %ecx,%edx
>    bf586:	09 d0                	or     %edx,%eax
>    bf588:	0f 84 e3 01 00 00    	je     bf771 <...>
> 
> to:
> 
>    bf572:	e8 fc ff ff ff       	call   cmpxchg8b_emu
>    bf577:	0f 84 b6 01 00 00    	je     bf733 <...>
> 
> No functional changes intended.

Side note: while there's no hard-written rule for it, I tend to use the 'no 
functional changes intended' line for pure identity transformations - which 
this one isn't, as it changes code generation materially.

So I removed that line - the explanation of the patch is clear enough IMO.

Thanks,

	Ingo