Re: [PATCH v2 3/6] powerpc/module: Optimise nearby branches in ELF V2 ABI stub

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

Le 26/09/2022 à 08:43, 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, 13 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index 4d816f7785b4..745ce9097dcf 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)
> @@ -429,6 +435,8 @@ static inline int create_stub(const Elf64_Shdr *sechdrs,
>   	long reladdr;
>   	func_desc_t desc;
>   	int i;
> +	u32 *jump_seq_addr = &entry->jump[PPC64_STUB_MTCTR_OFFSET];
> +	ppc_inst_t direct;
>   
>   	if (is_mprofile_ftrace_call(name))
>   		return create_ftrace_stub(entry, addr, me);
> @@ -439,6 +447,11 @@ static inline int create_stub(const Elf64_Shdr *sechdrs,
>   			return 0;
>   	}
>   
> +	/* Replace indirect branch sequence with direct branch where possible */
> +	if (!create_branch(&direct, jump_seq_addr, addr, 0))
> +		if (patch_instruction(jump_seq_addr, direct))

Why not use patch_branch() ?

> +			return 0;
> +
>   	/* Stub uses address relative to r2. */
>   	reladdr = (unsigned long)entry - my_r2(sechdrs, me);
>   	if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {