linux-kernel - Re: [PATCH 14/20] x86/barrier: Use alternative

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Message-ID: <zfabhk7c3fucov7lpfsqf5bj7iie5324ccgn4ingzzakoyhl4u@fzg364keuphn>
Date: Fri, 14 Mar 2025 17:05:34 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>, Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, 
	Uros Bizjak <ubizjak@...il.com>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 14/20] x86/barrier: Use alternative_io() in 32-bit
 barrier functions

On Fri, Mar 14, 2025 at 01:49:48PM -1000, Linus Torvalds wrote:
> So all of these patches look like good cleanups to me, but I do wonder
> if we should
> 
>  (a) not use some naming *quite* as generic as 'ARG()'
> 
>  (b) make the asms use ARG_OUT/ARG_IN/ARG_CLOBBER() to clarify
> 
> because that ARG(), ARG(), ARGC() pattern looks odd to me.
> 
> Maybe it's just me.
> 
> Regardless, I do think the series looks like a nice improvement even
> in the current form, even if that particular repeated pattern feels
> strange.

So originally I had ASM_OUTPUT/ASM_INPUT/ASM_CLOBBER, but I ended up
going with ARG() due to its nice vertical alignment and conciseness:


	__asm_call(qual,						\
		ALTERNATIVE("call this_cpu_cmpxchg8b_emu",		\
			    "cmpxchg8b " __percpu_arg([var]),		\
			    X86_FEATURE_CX8),				\
		ARG([var] "+m" (__my_cpu_var(_var)), "+a" (old__.low),	\
		    "+d" (old__.high)),					\
		ARG("b" (new__.low), "c" (new__.high), "S" (&(_var))),	\
		ARG("memory"));						\


Though ASM_OUTPUT/ASM_INPUT/ASM_CLOBBER isn't so bad either:

	__asm_call(qual,						\
		ALTERNATIVE("call this_cpu_cmpxchg8b_emu",		\
			    "cmpxchg8b " __percpu_arg([var]),		\
			    X86_FEATURE_CX8),				\
		ASM_OUTPUT([var] "+m" (__my_cpu_var(_var)),		\
			   "+a" (old__.low), "+d" (old__.high)),	\
		ASM_INPUT("b" (new__.low), "c" (new__.high),		\
			  "S" (&(_var))),				\
		ASM_CLOBBER("memory"));					\


That has the nice benefit of being more self-documenting, albeit more
verbose and less vertically aligned.

So I could go either way, really.

-- 
Josh