linux-kernel - Re: [PATCH -tip 0/3] x86/asm: Use generic asm operand modifiers instead of %P in asm templates

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Date: Tue, 19 Mar 2024 13:45:24 +0100
From: Borislav Petkov <bp@...en8.de>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...nel.org>, Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH -tip 0/3] x86/asm: Use generic asm operand modifiers
 instead of %P in asm templates

On Tue, Mar 19, 2024 at 11:40:11AM +0100, Uros Bizjak wrote:
> The "P" asm operand modifier is a x86 target-specific modifier.
> 
> For x86_64, when used with a symbol reference, the "P" modifier
> emits "sym" instead of "sym(%rip)". When used with a constant, the
> "P" modifier emits "cst" instead of "$cst". This property is used to
> emit bare symbol references and bare constants without all
> syntax-specific prefixes.
> 
> The generic "c", "n" and "a" operand modifiers should be used instead.
> The following table shows the modifiers supported by all targets and
> their effects:
> 
> Modifier    Description
> -----------------------------------------------------------
> 'c'         Require a constant operand and print the
>             constant expression with no punctuation.
> 'n'         Like '%c' except that the value of the constant
>             is negated before printing.
> 'a'         Substitute a memory reference, with the actual
>             operand treated as the address.  This may be
>             useful when outputting a "load address"
>             instruction, because often the assembler syntax
>             for such an instruction requires you to write
>             the operand as if it were a memory reference.
> 
> Also note that unlike GCC, clang emits %rip-relative symbol
> reference with "P" asm operand modifier, so the patch also unifies
> symbol handling with both compilers.

FTR, I really appreciate the clear explanations of the operand modifiers
along with an example in the commit messages. What they really do and
what they mean have caused some serious head-scratching in the past, up
to the point where I went through gcc sources with Matz' help to figure
out what some of them do.

So thanks!

:-)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette