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 09:09:51 -0400
From: Brian Gerst <brgerst@...il.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Andy Lutomirski <luto@...nel.org>, 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 6:44 AM Uros Bizjak <ubizjak@...il.com> 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.
>
> No functional changes intended.
>
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Brian Gerst <brgerst@...il.com>
> Cc: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
>
> Uros Bizjak (3):
>   x86/asm: Remove %P operand modifier from altinstr asm templates
>   x86/asm: Use %c/%n instead of %P operand modifier in asm templates
>   x86/asm: Use %a instead of %P operand modifier in asm templates
>
>  arch/x86/boot/main.c                 |  4 ++--
>  arch/x86/include/asm/alternative.h   | 22 +++++++++++-----------
>  arch/x86/include/asm/apic.h          |  2 +-
>  arch/x86/include/asm/atomic64_32.h   |  2 +-
>  arch/x86/include/asm/cpufeature.h    |  4 ++--
>  arch/x86/include/asm/irq_stack.h     |  2 +-
>  arch/x86/include/asm/processor.h     |  6 +++---
>  arch/x86/include/asm/special_insns.h |  4 ++--
>  arch/x86/include/asm/uaccess.h       |  4 ++--
>  9 files changed, 25 insertions(+), 25 deletions(-)
>
> --
> 2.44.0
>

My one concern is does this work with older compilers?  Technically,
GCC 5.1 is still supported.  I seem to recall some issues around these
modifiers in the past, but don't remember any details.  Otherwise,
it's a nice cleanup.


Brian Gerst