vmspawn: extend kernel cmdline with extra args

This changes how the "extra" command line arguments passed to vmspawn
are handled.

Previously they were appended to the QEMU command line directly.
Now they are appended to the kernel command line using SMBIOS instead.

man/systemd-vmspawn.xml

@@ -39,7 +39,7 @@
   <refsect1>
     <title>Options</title>
 
-    <para>The arguments are passed straight through to QEMU, extending its command line arguments.</para>
+    <para>The excess arguments are passed as extra kernel command line arguments using SMBIOS.</para>
 
     <para>The following options are understood:</para>
 

src/vmspawn/vmspawn.c

@@ -211,7 +211,7 @@ static int parse_argv(int argc, char *argv[]) {
 static int run_virtual_machine(void) {
         _cleanup_(ovmf_config_freep) OvmfConfig *ovmf_config = NULL;
         _cleanup_strv_free_ char **cmdline = NULL;
-        _cleanup_free_ char *machine = NULL, *qemu_binary = NULL, *mem = NULL;
+        _cleanup_free_ char *machine = NULL, *qemu_binary = NULL, *mem = NULL, *kcl = NULL;
         int r;
 
         bool use_kvm = arg_qemu_kvm > 0;
@@ -357,9 +357,23 @@ static int run_virtual_machine(void) {
         if (r < 0)
                 return log_oom();
 
-        r = strv_extend_strv(&cmdline, arg_parameters, false);
-        if (r < 0)
-                return log_oom();
+        if (strv_length(arg_parameters) != 0) {
+#if ARCHITECTURE_SUPPORTS_SMBIOS
+                kcl = strv_join(arg_parameters, " ");
+                if (!kcl)
+                        return log_oom();
+
+                r = strv_extend(&cmdline, "-smbios");
+                if (r < 0)
+                        return log_oom();
+
+                r = strv_extendf(&cmdline, "type=11,value=io.systemd.stub.kernel-cmdline-extra=%s", kcl);
+                if (r < 0)
+                        return log_oom();
+#else
+                log_warning("Cannot append extra args to kernel cmdline, native architecture doesn't support SMBIOS");
+#endif
+        }
 
         pid_t child_pid;
         r = safe_fork(qemu_binary, 0, &child_pid);