[PATCH 03/10] ARM: assembler: introduce bl_r and bl_m macros
From: Ard Biesheuvel <ardb@kernel.org>
Date: 2021-09-21 09:57:07
Subsystem:
arm port, the rest · Maintainers:
Russell King, Linus Torvalds
Add some macros that abstract the difference between the ways indirect calls are performed on older and newer ARM architecture revisions. The main difference is to prefer blx instructions over explicit LR assignments when possible, as these tend to confuse the prediction logic in out-of-order cores when speculating across a function return. Signed-off-by: Ard Biesheuvel <ardb@kernel.org> --- arch/arm/include/asm/assembler.h | 37 ++++++++++++++++++++ 1 file changed, 37 insertions(+)
diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index 7d23d4bb2168..abb8202ef0da 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h@@ -624,4 +624,41 @@ THUMB( orr \reg , \reg , #PSR_T_BIT ) .endif .endm + /* + * bl_r - branch and link to register + * + * @dst: target to branch to + * @c: conditional opcode suffix + */ + .macro bl_r, dst:req, c + .if __LINUX_ARM_ARCH__ < 6 + mov\c lr, pc + mov\c pc, \dst + .else + blx\c \dst + .endif + .endm + + /* + * bl_m - branch and link to address held in memory + * + * @dst: memory reference to load the branch target from + * + * This uses the same syntax as ldr instructions, including things like + * register writeback (where it makes sense). For example, + * + * bl_m [ip, #<offset>] + * + * Note that @dst should not reference lr as it may be clobbered early. + */ + .macro bl_m, dst:vararg + .if __LINUX_ARM_ARCH__ < 6 + mov lr, pc + ldr pc, \dst + .else + ldr lr, \dst + blx lr + .endif + .endm + #endif /* __ASM_ASSEMBLER_H__ */
--
2.30.2
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