baremetal: allow arbitrary exit status with the magic string

test-baremetal: fix missing setting x0 return value

Examples were just returning on ret without setting x0, which led to
failures... those were not noticed because of how broken the testing system
was ;-)

common.py

@@ -115,7 +115,7 @@ consts['userland_out_exts'] = [
     consts['obj_ext'],
 ]
 consts['default_config_file'] = os.path.join(consts['data_dir'], 'config.py')
-consts['magic_fail_string'] = b'lkmc_test_fail'
+consts['serial_magic_exit_status_regexp_string'] = b'lkmc_exit_status_(\d+)'
 consts['baremetal_lib_basename'] = 'lib'
 consts['emulator_userland_only_short_to_long_dict'] = collections.OrderedDict([
     ('n', 'native'),
@@ -482,6 +482,25 @@ CLI arguments to pass to the userland executable.
         )
 
         # Run.
+        self.add_argument(
+            '--background',
+            default=False,
+            help='''\
+Make programs that would take over the terminal such as QEMU full system run on the
+background instead.
+
+Currently only implemented for ./run.
+
+Interactive input cannot be given.
+
+Send QEMU serial output to a file instead of the host terminal.
+
+TODO: use a port instead. If only there was a way to redirect a serial to multiple
+places, both to a port and a file? We use the file currently to be able to have
+any output at all.
+https://superuser.com/questions/1373226/how-to-redirect-qemu-serial-output-to-both-a-file-and-the-terminal-or-a-port
+'''
+        )
         self.add_argument(
             '--in-tree',
             default=False,
@@ -763,7 +782,10 @@ Incompatible archs are skipped.
             env['executable'] = env['qemu_executable']
             env['run_dir'] = env['qemu_run_dir']
             env['termout_file'] = env['qemu_termout_file']
-            env['guest_terminal_file'] = env['qemu_termout_file']
+            if env['background']:
+                env['guest_terminal_file'] = env['qemu_background_serial_file']
+            else:
+                env['guest_terminal_file'] = env['qemu_termout_file']
             env['trace_txt_file'] = env['qemu_trace_txt_file']
         env['run_cmd_file'] = join(env['run_dir'], 'run.sh')
 
@@ -1353,7 +1375,6 @@ https://github.com/cirosantilli/linux-kernel-module-cheat#gem5-debug-build
             **self._build_arguments[argument_name]
         )
 
-
     def _build_one(
         self,
         in_path,