[PATCH v3 3/6] powerpc/module: Optimise nearby branches in ELF V2 ABI stub

[Christophe Leroy]

Le 05/10/2022 à 07:32, 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 at linux.ibm.com>

Reviewed-by: Christophe Leroy <christophe.leroy at csgroup.eu>

> ---
>   arch/powerpc/kernel/module_64.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index 4d816f7785b4..13ce7a4d8a8d 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -141,6 +141,12 @@ static u32 ppc64_stub_insns[] = {
>   	PPC_RAW_BCTR(),
>   };
>   
> +#ifdef CONFIG_PPC64_ELF_ABI_V1
> +#define PPC64_STUB_MTCTR_OFFSET 5
> +#else
> +#define PPC64_STUB_MTCTR_OFFSET 4
> +#endif
> +
>   /* Count how many different 24-bit relocations (different symbol,
>      different addend) */
>   static unsigned int count_relocs(const Elf64_Rela *rela, unsigned int num)
> @@ -426,6 +432,7 @@ static inline int create_stub(const Elf64_Shdr *sechdrs,
>   			      struct module *me,
>   			      const char *name)
>   {
> +	int err;
>   	long reladdr;
>   	func_desc_t desc;
>   	int i;
> @@ -439,6 +446,11 @@ static inline int create_stub(const Elf64_Shdr *sechdrs,
>   			return 0;
>   	}
>   
> +	/* Replace indirect branch sequence with direct branch where possible */
> +	err = patch_branch(&entry->jump[PPC64_STUB_MTCTR_OFFSET], addr, 0);
> +	if (err && err != -ERANGE)
> +		return 0;
> +
>   	/* Stub uses address relative to r2. */
>   	reladdr = (unsigned long)entry - my_r2(sechdrs, me);
>   	if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {