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

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Message-ID: <CAMj1kXEWm9WuvxYN8ks09m3Rv+pt1Hd7=DcXuTwm+Etw2Y-KOQ@mail.gmail.com>
Date:   Thu, 6 Oct 2022 10:17:44 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
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>,
        Mark Rutland <mark.rutland@....com>,
        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, 6 Oct 2022 at 10:05, Jisheng Zhang <jszhang@...nel.org> 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."
>
> Tested with building a simple external kernel module with "O0".
>
> [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"

Is this still necessary if we specify the constraints in a more
reasonable manner:

 "      .quad           %c0 - . + %1            \n\t"
 :  :  "i"(key), "i"(branch) :  : l_yes);

instead of the horrid hack with the char* cast and using a bool as an
array index?




> @@ -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
>