arm64: scs: Use 'scs_sp' register alias for x18

x18 holds the SCS stack pointer value, so introduce a register alias to
make this easier to read in assembly code.
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/scs.h

@@ -7,12 +7,14 @@
 #include <asm/asm-offsets.h>
 
 #ifdef CONFIG_SHADOW_CALL_STACK
+	scs_sp	.req	x18
+
 	.macro scs_load tsk, tmp
-	ldr	x18, [\tsk, #TSK_TI_SCS_SP]
+	ldr	scs_sp, [\tsk, #TSK_TI_SCS_SP]
 	.endm
 
 	.macro scs_save tsk, tmp
-	str	x18, [\tsk, #TSK_TI_SCS_SP]
+	str	scs_sp, [\tsk, #TSK_TI_SCS_SP]
 	.endm
 #else
 	.macro scs_load tsk, tmp

arch/arm64/kernel/entry.S

@@ -394,7 +394,7 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
 	.macro	irq_stack_entry
 	mov	x19, sp			// preserve the original sp
 #ifdef CONFIG_SHADOW_CALL_STACK
-	mov	x24, x18		// preserve the original shadow stack
+	mov	x24, scs_sp		// preserve the original shadow stack
 #endif
 
 	/*
@@ -416,7 +416,7 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
 
 #ifdef CONFIG_SHADOW_CALL_STACK
 	/* also switch to the irq shadow stack */
-	adr_this_cpu x18, irq_shadow_call_stack, x26
+	adr_this_cpu scs_sp, irq_shadow_call_stack, x26
 #endif
 
 9998:
@@ -430,7 +430,7 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
 	.macro	irq_stack_exit
 	mov	sp, x19
 #ifdef CONFIG_SHADOW_CALL_STACK
-	mov	x18, x24
+	mov	scs_sp, x24
 #endif
 	.endm
 
@@ -1071,9 +1071,9 @@ SYM_CODE_START(__sdei_asm_handler)
 #ifdef CONFIG_SHADOW_CALL_STACK
 	/* Use a separate shadow call stack for normal and critical events */
 	cbnz	w4, 3f
-	adr_this_cpu dst=x18, sym=sdei_shadow_call_stack_normal, tmp=x6
+	adr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_normal, tmp=x6
 	b	4f
-3:	adr_this_cpu dst=x18, sym=sdei_shadow_call_stack_critical, tmp=x6
+3:	adr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_critical, tmp=x6
 4:
 #endif
 

arch/arm64/kernel/head.S

@@ -426,7 +426,7 @@ SYM_FUNC_START_LOCAL(__primary_switched)
 	mov	x29, sp
 
 #ifdef CONFIG_SHADOW_CALL_STACK
-	adr_l	x18, init_shadow_call_stack	// Set shadow call stack
+	adr_l	scs_sp, init_shadow_call_stack	// Set shadow call stack
 #endif
 
 	str_l	x21, __fdt_pointer, x5		// Save FDT pointer