Re: [PATCH 2/4] s390/jump_label: Implement ARCH_STATIC_BRANCH_JUMP_ASM and ARCH_STATIC_BRANCH_ASM macros

["Gary Guo" <gary@garyguo.net>]

On Tue May 12, 2026 at 11:59 AM BST, Jan Polensky wrote:
> Rust static branch support needs the s390 jump label instruction sequence
> and __jump_table emission in a reusable form. The current implementation
> embeds the sequence directly in the C asm goto blocks, which cannot be
> shared with Rust.
>
> Introduce ARCH_STATIC_BRANCH_ASM and ARCH_STATIC_BRANCH_JUMP_ASM to
> describe the brcl sequences for the likely-false and likely-true cases
> and to emit the same __jump_table entries as before. Switch the existing
> C helpers to use the new macros to avoid duplication without changing
> the generated code.
>
> Signed-off-by: Jan Polensky <japo@linux.ibm.com>
> ---
>  arch/s390/include/asm/jump_label.h | 33 +++++++++++++++++-------------
>  1 file changed, 19 insertions(+), 14 deletions(-)
>
> diff --git a/arch/s390/include/asm/jump_label.h b/arch/s390/include/asm/jump_label.h
> index d9cbc18f6b2e..1f0114150786 100644
> --- a/arch/s390/include/asm/jump_label.h
> +++ b/arch/s390/include/asm/jump_label.h
> @@ -23,15 +23,25 @@
>   * We use a brcl 0,<offset> instruction for jump labels so it
>   * can be easily distinguished from a hotpatch generated instruction.
>   */

The comment looks out of place now, you probably want to move
ARCH_JUMP_TABLE_ENTRY above it.

Best,
Gary

> +#define ARCH_JUMP_TABLE_ENTRY(key, label, local_label)	\
> +	".pushsection __jump_table,\"aw\"\n"		\
> +	".balign	8\n"				\
> +	".long	" local_label "-.," label "-.\n"	\
> +	".quad	" key "-.\n"				\
> +	".popsection\n"
> +
> +#define ARCH_STATIC_BRANCH_ASM(key, label)	\
> +	"0:	brcl 0," label "\n"		\
> +	ARCH_JUMP_TABLE_ENTRY(key, label, "0b")
> +
> +#define ARCH_STATIC_BRANCH_JUMP_ASM(key, label)	\
> +	"0:	brcl 15," label "\n"		\
> +	ARCH_JUMP_TABLE_ENTRY(key, label, "0b")
> +
>  static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
>  {
> -	asm goto("0:	brcl 0,%l[label]\n"
> -			  ".pushsection __jump_table,\"aw\"\n"
> -			  ".balign	8\n"
> -			  ".long	0b-.,%l[label]-.\n"
> -			  ".quad	%0+%1-.\n"
> -			  ".popsection\n"
> -			  : : JUMP_LABEL_STATIC_KEY_CONSTRAINT (key), "i" (branch) : : label);
> +	asm goto(ARCH_STATIC_BRANCH_ASM("%0+%1", "%l[label]")
> +		: : JUMP_LABEL_STATIC_KEY_CONSTRAINT (key), "i" (branch) : : label);
>  	return false;
>  label:
>  	return true;
> @@ -39,13 +49,8 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
>  
>  static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
>  {
> -	asm goto("0:	brcl 15,%l[label]\n"
> -			  ".pushsection __jump_table,\"aw\"\n"
> -			  ".balign	8\n"
> -			  ".long	0b-.,%l[label]-.\n"
> -			  ".quad	%0+%1-.\n"
> -			  ".popsection\n"
> -			  : : JUMP_LABEL_STATIC_KEY_CONSTRAINT (key), "i" (branch) : : label);
> +	asm goto(ARCH_STATIC_BRANCH_JUMP_ASM("%0+%1", "%l[label]")
> +		: : JUMP_LABEL_STATIC_KEY_CONSTRAINT (key), "i" (branch) : : label);
>  	return false;
>  label:
>  	return true;