ARM: assembler: introduce bl_r macro

Add a bl_r macro 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>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Keith Packard <keithpac@amazon.com>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M

arch/arm/include/asm/assembler.h

@@ -624,4 +624,19 @@ 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
+
 #endif /* __ASM_ASSEMBLER_H__ */