[PATCH v4 3/6] powerpc/module: Optimise nearby branches in ELF V2 ABI stub

[Benjamin Gray]

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>
Reviewed-by: Andrew Donnellan <ajd at linux.ibm.com>
---
 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 83a6f6e22e3b..3a90043b006c 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)) {
-- 
2.37.3