arm64: Add macro version of the BTI instruction

BTI is only available from v8.5 so we need to encode it using HINT in
generic code and for older toolchains. Add an assembler macro based on
one written by Mark Rutland which lets us use the mnemonic and update
the existing users.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20211214152714.2380849-2-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: broonie

arch/arm64/crypto/aes-modes.S

@@ -363,15 +363,15 @@ ST5(	mov		v4.16b, vctr.16b		)
 	adr		x16, 1f
 	sub		x16, x16, x12, lsl #3
 	br		x16
-	hint		34			// bti c
+	bti		c
 	mov		v0.d[0], vctr.d[0]
-	hint		34			// bti c
+	bti		c
 	mov		v1.d[0], vctr.d[0]
-	hint		34			// bti c
+	bti		c
 	mov		v2.d[0], vctr.d[0]
-	hint		34			// bti c
+	bti		c
 	mov		v3.d[0], vctr.d[0]
-ST5(	hint		34				)
+ST5(	bti		c				)
 ST5(	mov		v4.d[0], vctr.d[0]		)
 1:	b		2f
 	.previous

arch/arm64/include/asm/assembler.h

@@ -790,6 +790,16 @@ alternative_endif
 .Lnoyield_\@:
 	.endm
 
+/*
+ * Branch Target Identifier (BTI)
+ */
+	.macro  bti, targets
+	.equ	.L__bti_targets_c, 34
+	.equ	.L__bti_targets_j, 36
+	.equ	.L__bti_targets_jc,38
+	hint	#.L__bti_targets_\targets
+	.endm
+
 /*
  * This macro emits a program property note section identifying
  * architecture features which require special handling, mainly for

arch/arm64/include/asm/linkage.h

@@ -6,12 +6,7 @@
 
 #if defined(CONFIG_ARM64_BTI_KERNEL) && defined(__aarch64__)
 
-/*
- * Since current versions of gas reject the BTI instruction unless we
- * set the architecture version to v8.5 we use the hint instruction
- * instead.
- */
-#define BTI_C hint 34 ;
+#define BTI_C bti c ;
 
 /*
  * When using in-kernel BTI we need to ensure that PCS-conformant assembly