Re: [PATCH v3 3/6] powerpc/module: Optimise nearby branches in ELF V2 ABI stub
From: Andrew Donnellan <hidden>
Date: 2022-10-06 08:26:17
On Wed, 2022-10-05 at 16:32 +1100, Benjamin Gray wrote:
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 <redacted>
I'm not well versed in this code but nothing stands out as problematic and it makes sense. Reviewed-by: Andrew Donnellan <redacted>
quoted hunk ↗ jump to hunk
--- arch/powerpc/kernel/module_64.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)diff --git a/arch/powerpc/kernel/module_64.cb/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)) {
-- Andrew Donnellan OzLabs, ADL Canberra ajd@linux.ibm.com IBM Australia Limited