baremetal: aarch64 semihosting exit

baremetal/arch/aarch64/semihost_exit.S

@@ -0,0 +1,20 @@
+.global main
+main:
+    /* 0x20026 == ADP_Stopped_ApplicationExit */
+    mov x1, #0x26
+    movk x1, #2, lsl #16
+    str x1, [sp,#0]
+
+    /* Exit status code. Host QEMU process exits with that status. */
+    mov x0, #0
+    str x0, [sp,#8]
+
+    /* x1 contains the address of parameter block.
+     * Any memory address could be used. */
+    mov x1, sp
+
+    /* SYS_EXIT */
+    mov w0, #0x18
+
+    /* Do the semihosting call on A64. */
+    hlt 0xf000

baremetal/arch/arm/semihost_exit.S

@@ -1,5 +1,8 @@
 .global main
 main:
+    /* SYS_EXIT */
     mov r0, #0x18
+    /* ADP_Stopped_ApplicationExit */
     ldr r1, =#0x20026
+    /* Do the semihosting call on A32. */
     svc 0x00123456

baremetal/lib/common.c

@@ -58,8 +58,21 @@ int _write(int file, char *ptr, int len) {
     return len;
 }
 
+/* Only 0 is supported for now, arm semihosting cannot handle it. */
 void _exit(int status) {
 #if defined(__arm__)
     __asm__ __volatile__ ("mov r0, #0x18; ldr r1, =#0x20026; svc 0x00123456");
+#elif defined(__aarch64__)
+	/* TODO actually use the exit value here, just for fun. */
+    __asm__ __volatile__ (
+		"mov x1, #0x26\n" \
+		"movk x1, #2, lsl #16\n" \
+		"str x1, [sp,#0]\n" \
+		"mov x0, #0\n" \
+		"str x0, [sp,#8]\n" \
+		"mov x1, sp\n" \
+		"mov w0, #0x18\n" \
+		"hlt 0xf000\n"
+    );
 #endif
 }