Re: [PATCH 3/6] powerpc/module: Optimise nearby branches in ELF V2 ABI stub

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 16/09/2022 à 08:23, Benjamin Gray a écrit :
> Inserts a direct branch to the stub target when possible, replacing the
> mtctr/btctr sequence.
> 
> The load into r12 could potentially be skipped too, but that change
> would need to refactor the arguments to indicate that the address
> does not have a separate local entry point.
> 
> This helps the static call implementation, where modules calling their
> own trampolines are called through this stub and the trampoline is
> easily within range of a direct branch.
> 
> Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
> ---
>   arch/powerpc/kernel/module_64.c | 13 +++++++++++--
>   1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index 3656476097c2..03ab28d86008 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -432,8 +432,17 @@ static inline int create_stub(const Elf64_Shdr *sechdrs,
>   		return create_ftrace_stub(entry, addr, me);
>   
>   	for (i = 0; i < ARRAY_SIZE(ppc64_stub_insns); i++) {
> -		if (patch_instruction(&entry->jump[i],
> -				      ppc_inst(ppc64_stub_insns[i])))
> +		ppc_inst_t inst = ppc_inst(ppc64_stub_insns[i]);
> +
> +		// Replace the indirect branch with a direct branch where possible
> +		if (IS_ENABLED(CONFIG_PPC64_ELF_ABI_V2) && i == 4) {
> +			ppc_inst_t direct;

Add blank line here.

> +			if (create_branch(&direct, (void*) entry + (i * 4), addr, 0) == 0) {

No braces for single lines if statement.

> +				inst = direct;
> +			}
> +		}
> +
> +		if (patch_instruction(&entry->jump[i], inst))
>   			return 0;
>   	}
>