coroutine/arm64: Skip saving/restoring x30 twice and use autiasp

We don't need to save/restore x30 twice, and we can just use `ret`,
which uses x30 as return address register instead of explicit `ret <reg>`
instruction. This also allows us to use `autiasp` instead of `autia1716`
and we can skip setting SP/LR to x16/x17.

Also the size of register save area is shrunk by 16 bytes due to the
removal of extra x30 save/restore.

coroutine/arm64/Context.S

@@ -36,7 +36,7 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 	hint #34
 #endif
 	# Make space on the stack for caller registers
-	sub sp, sp, 0xb0
+	sub sp, sp, 0xa0
 
 	# Save caller registers
 	stp d8, d9, [sp, 0x00]
@@ -50,9 +50,6 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 	stp x27, x28, [sp, 0x80]
 	stp x29, x30, [sp, 0x90]
 
-	# Save return address
-	str x30, [sp, 0xa0]
-
 	# Save stack pointer to x0 (first argument)
 	mov x2, sp
 	str x2, [x0, 0]
@@ -73,20 +70,16 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 	ldp x27, x28, [sp, 0x80]
 	ldp x29, x30, [sp, 0x90]
 
-	# Load return address into x17
-	ldr x17, [sp, 0xa0]
-
 	# Pop stack frame
-	add sp, sp, 0xb0
+	add sp, sp, 0xa0
 
 #if defined(__ARM_FEATURE_PAC_DEFAULT) && (__ARM_FEATURE_PAC_DEFAULT != 0)
-	mov x16, sp
-	# autia1716
-	hint #12
+	# autiasp: Authenticate x30 (LR) with SP and key A
+	hint #29
 #endif
 
-	# Jump to return address (in x17)
-	ret x17
+	# Jump to return address (in x30)
+	ret
 
 #if defined(__linux__) && defined(__ELF__)
 .section .note.GNU-stack,"",%progbits

coroutine/arm64/Context.h

@@ -17,7 +17,7 @@
 
 #define COROUTINE __attribute__((noreturn)) void
 
-enum {COROUTINE_REGISTERS = 0xb0 / 8};
+enum {COROUTINE_REGISTERS = 0xa0 / 8};
 
 #if defined(__SANITIZE_ADDRESS__)
     #define COROUTINE_SANITIZE_ADDRESS
@@ -89,7 +89,7 @@ static inline void coroutine_initialize(
     context->stack_pointer -= COROUTINE_REGISTERS;
     memset(context->stack_pointer, 0, sizeof(void*) * COROUTINE_REGISTERS);
 
-    context->stack_pointer[0xa0 / 8] = ptrauth_sign_instruction_addr((void*)start, (void*)top);
+    context->stack_pointer[0x98 / 8] = ptrauth_sign_instruction_addr((void*)start, (void*)top);
 }
 
 struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target);