linux-kernel - Re: [PATCH 1/2] arm64: jump_label: mark arguments as const to satisfy asm constraints

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Message-ID: <Yz6qksBFFj9Wo9M8@FVFF77S0Q05N.cambridge.arm.com>
Date:   Thu, 6 Oct 2022 11:14:42 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Jisheng Zhang <jszhang@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        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>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64: jump_label: mark arguments as const to
 satisfy asm constraints

On Thu, Oct 06, 2022 at 03:55:41PM +0800, Jisheng Zhang wrote:
> Inspired by x86 commit 864b435514b2("x86/jump_label: Mark arguments as
> const to satisfy asm constraints"), mark arch_static_branch()'s and
> arch_static_branch_jump()'s arguments as const to satisfy asm
> constraints. And Steven in [1] also pointed out that "The "i"
> constraint needs to be a constant."

It needs to be a *compile-time constant*, but `const` on a function argument
only ensures that the function can't modify the argument, not that it's a
constant in the caller.

I think this is a quirk of the optimizer rather than anything else.

> Tested with building a simple external kernel module with "O0".

Is building with `-O0` supported? I thought we required using `-O2` or above
for a bunch of code that requires constant propagation, etc.

I don't really have a problem with making this const, but I don't particularly
want to try to "fix" all the other code that depends on constant propagation to
assemble, and I'm worried this is the canary in the coal mine.

Thanks,
Mark.

> 
> [1]https://lore.kernel.org/all/20210212094059.5f8d05e8@gandalf.local.home/
> 
> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> ---
>  arch/arm64/include/asm/jump_label.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
> index cea441b6aa5d..48ddc0f45d22 100644
> --- a/arch/arm64/include/asm/jump_label.h
> +++ b/arch/arm64/include/asm/jump_label.h
> @@ -15,8 +15,8 @@
>  
>  #define JUMP_LABEL_NOP_SIZE		AARCH64_INSN_SIZE
>  
> -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(
>  		"1:	nop					\n\t"
> @@ -32,8 +32,8 @@ 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(
>  		"1:	b		%l[l_yes]		\n\t"
> -- 
> 2.37.2
>