baremetal: aarch64 semihosting exit

README.adoc

@@ -819,6 +819,11 @@ auto_reset_addr_64 = True
 
 That patch breaks `--arch arm`, so don't forget to remove it if you want to go back to it.
 
+When doing bare metal programming, it is likely that you will want to learn assembly language basics. Have a look at these tutorials for the userland part:
+
+* https://github.com/cirosantilli/x86-assembly-cheat
+* https://github.com/cirosantilli/arm-assembly-cheat
+
 For more information on baremetal, see the section: <<baremetal>>. The following subjects are particularly important:
 
 * <<tracing>>

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
 }

build-all

@@ -22,7 +22,7 @@ for arch in $archs; do
   if [ ! "$arch" = x86_64 ]; then
     ./build-crosstool-ng --arch "$arch"
     ./build-baremetal --arch "$arch"
-    ./build-baremetal --arch "$arch" -g
-    ./build-baremetal --arch "$arch" -g --machine RealViewPBX
+    ./build-baremetal --arch "$arch" --gem5
+    ./build-baremetal --arch "$arch" --gem5 --machine RealViewPBX
   fi
 done

build-baremetal

@@ -119,16 +119,20 @@ def main(args, extra_args=None):
             bootloader_obj=bootloader_obj,
             common_obj=common_obj,
         )
+        arch_dir = os.path.join('arch', args.arch)
+        if os.path.isdir(os.path.join(common.baremetal_src_dir, arch_dir)):
             build_dir(
-            os.path.join('arch', args.arch),
+                arch_dir,
                 gcc=gcc,
                 cflags=cflags,
                 entry_address=entry_address,
                 bootloader_obj=bootloader_obj,
                 common_obj=common_obj,
             )
+        arch_dir = os.path.join('arch', args.arch, 'no_bootloader')
+        if os.path.isdir(os.path.join(common.baremetal_src_dir, arch_dir)):
             build_dir(
-            os.path.join('arch', args.arch, 'no_bootloader'),
+                arch_dir,
                 gcc=gcc,
                 cflags=cflags,
                 entry_address=entry_address,

run

@@ -271,7 +271,7 @@ def main(args, extra_args=None):
                 '-device', 'edu',
             ])
         elif args.arch == 'arm' or args.arch == 'aarch64':
-            extra_qemu_args.append('-semihosting')
+            extra_emulator_args.append('-semihosting')
             if args.kgdb:
                 kernel_cli += ' kgdboc=ttyAMA0,115200'
             if args.arch == 'arm':