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

From: Ard Biesheuvel
Date: Thu Oct 06 2022 - 04:18:08 EST


On Thu, 6 Oct 2022 at 10:05, Jisheng Zhang <jszhang@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx/
>
> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> ---
> 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
>