linux-kernel - Re: [PATCH] riscv: jump_label: mark arguments as const to satisfy asm constraints

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Date:   Thu, 6 Oct 2022 14:41:44 +0200
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Jisheng Zhang <jszhang@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Samuel Holland <samuel@...lland.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] riscv: jump_label: mark arguments as const to satisfy
 asm constraints

On Thu, Oct 06, 2022 at 02:40:28PM +0800, Jisheng Zhang wrote:
> Samuel reported that the static branch usage in cpu_relax() breaks
> building with CONFIG_CC_OPTIMIZE_FOR_SIZE[1]:
> 
> In file included from <command-line>:
> ./arch/riscv/include/asm/jump_label.h: In function 'cpu_relax':
> ././include/linux/compiler_types.h:285:33: warning: 'asm' operand 0
> probably does not match constraints
>   285 | #define asm_volatile_goto(x...) asm goto(x)
>       |                                 ^~~
> ./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro
> 'asm_volatile_goto'
>    41 |         asm_volatile_goto(
>       |         ^~~~~~~~~~~~~~~~~
> ././include/linux/compiler_types.h:285:33: error: impossible constraint
> in 'asm'
>   285 | #define asm_volatile_goto(x...) asm goto(x)
>       |                                 ^~~
> ./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro
> 'asm_volatile_goto'
>    41 |         asm_volatile_goto(
>       |         ^~~~~~~~~~~~~~~~~
> make[1]: *** [scripts/Makefile.build:249:
> arch/riscv/kernel/vdso/vgettimeofday.o] Error 1
> make: *** [arch/riscv/Makefile:128: vdso_prepare] Error 2
> 
> Maybe "-Os" prevents GCC from detecting that the key/branch arguments
> can be treated as constants and used as immediate operands. Inspired
> by x86's commit 864b435514b2("x86/jump_label: Mark arguments as const to
> satisfy asm constraints"), and as pointed out by Steven in [2] "The "i"
> constraint needs to be a constant.", let's do similar modifications to
> riscv.
> 
> Tested by CC_OPTIMIZE_FOR_SIZE + gcc and CC_OPTIMIZE_FOR_SIZE + clang.
> 
> [1]https://lore.kernel.org/linux-riscv/20220922060958.44203-1-samuel@sholland.org/
> [2]https://lore.kernel.org/all/20210212094059.5f8d05e8@gandalf.local.home/
> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> ---
>  arch/riscv/include/asm/jump_label.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/jump_label.h b/arch/riscv/include/asm/jump_label.h
> index 38af2ec7b9bf..6d58bbb5da46 100644
> --- a/arch/riscv/include/asm/jump_label.h
> +++ b/arch/riscv/include/asm/jump_label.h
> @@ -14,8 +14,8 @@
>  
>  #define JUMP_LABEL_NOP_SIZE 4
>  
> -static __always_inline bool arch_static_branch(struct static_key *key,
> -					       bool branch)
> +static __always_inline bool arch_static_branch(struct static_key * const key,
> +					       const bool branch)
>  {
>  	asm_volatile_goto(
>  		"	.option push				\n\t"
> @@ -35,8 +35,8 @@ static __always_inline bool arch_static_branch(struct static_key *key,
>  	return true;
>  }
>  
> -static __always_inline bool arch_static_branch_jump(struct static_key *key,
> -						    bool branch)
> +static __always_inline bool arch_static_branch_jump(struct static_key * const key,
> +						    const bool branch)
>  {
>  	asm_volatile_goto(
>  		"	.option push				\n\t"
> -- 
> 2.37.2
>

Reviewed-by: Andrew Jones <ajones@...tanamicro.com>