audio: extend -audio to allow creating a default backend

If "-audio BACKEND" is used without a model, the resulting backend
will be used whenever the audiodev property is not specified.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

audio/audio.c

@@ -1820,7 +1820,7 @@ bool AUD_register_card (const char *name, QEMUSoundCard *card, Error **errp)
             card->state = audio_init(NULL, errp);
             if (!card->state) {
                 if (!QSIMPLEQ_EMPTY(&audiodevs)) {
-                    error_append_hint(errp, "Perhaps you wanted to set audiodev=%s?\n",
+                    error_append_hint(errp, "Perhaps you wanted to use -audio or set audiodev=%s?\n",
                                       QSIMPLEQ_FIRST(&audiodevs)->dev->id);
                 }
                 return false;

qemu-options.hx

@@ -728,20 +728,22 @@ ERST
 
 
 DEF("audio", HAS_ARG, QEMU_OPTION_audio,
+    "-audio [driver=]driver[,prop[=value][,...]]\n"
+    "                specifies default audio backend when `audiodev` is not\n"
+    "                used to create a machine or sound device;"
+    "                options are the same as for -audiodev\n"
     "-audio [driver=]driver,model=value[,prop[=value][,...]]\n"
     "                specifies the audio backend and device to use;\n"
     "                apart from 'model', options are the same as for -audiodev.\n"
     "                use '-audio model=help' to show possible devices.\n",
     QEMU_ARCH_ALL)
 SRST
-``-audio [driver=]driver,model=value[,prop[=value][,...]]``
-    This option is a shortcut for configuring both the guest audio
-    hardware and the host audio backend in one go.
-    The driver option is the same as with the corresponding ``-audiodev`` option below.
-    The guest hardware model can be set with ``model=modelname``.
-
-    Use ``driver=help`` to list the available drivers,
-    and ``model=help`` to list the available device types.
+``-audio [driver=]driver[,model=value][,prop[=value][,...]]``
+    If the ``model`` option is specified, ``-audio`` is a shortcut
+    for configuring both the guest audio hardware and the host audio
+    backend in one go. The guest hardware model can be set with
+    ``model=modelname``.  Use ``model=help`` to list the available
+    device types.
 
     The following two example do exactly the same, to show how ``-audio``
     can be used to shorten the command line length:
@@ -750,6 +752,17 @@ SRST
 
         |qemu_system| -audiodev pa,id=pa -device sb16,audiodev=pa
         |qemu_system| -audio pa,model=sb16
+
+    If the ``model`` option is not specified, ``-audio`` is used to
+    configure a default audio backend that will be used whenever the
+    ``audiodev`` property is not set on a device or machine.  In
+    particular, ``-audio none`` ensures that no audio is produced even
+    for machines that have embedded sound hardware.
+
+    In both cases, the driver option is the same as with the corresponding
+    ``-audiodev`` option below.  Use ``driver=help`` to list the available
+    drivers.
+
 ERST
 
 DEF("audiodev", HAS_ARG, QEMU_OPTION_audiodev,

system/vl.c

@@ -2935,7 +2935,7 @@ void qemu_init(int argc, char **argv)
                 break;
             case QEMU_OPTION_audio: {
                 bool help;
-                char *model;
+                char *model = NULL;
                 Audiodev *dev = NULL;
                 Visitor *v;
                 QDict *dict = keyval_parse(optarg, "driver", &help, &error_fatal);
@@ -2948,22 +2948,25 @@ void qemu_init(int argc, char **argv)
                 if (!qdict_haskey(dict, "id")) {
                     qdict_put_str(dict, "id", "audiodev0");
                 }
-                if (!qdict_haskey(dict, "model")) {
-                    error_setg(&error_fatal, "Parameter 'model' is missing");
-                }
-                model = g_strdup(qdict_get_str(dict, "model"));
-                qdict_del(dict, "model");
-                if (is_help_option(model)) {
-                    show_valid_soundhw();
-                    exit(0);
+                if (qdict_haskey(dict, "model")) {
+                    model = g_strdup(qdict_get_str(dict, "model"));
+                    qdict_del(dict, "model");
+                    if (is_help_option(model)) {
+                        show_valid_soundhw();
+                        exit(0);
+                    }
                 }
                 v = qobject_input_visitor_new_keyval(QOBJECT(dict));
                 qobject_unref(dict);
                 visit_type_Audiodev(v, NULL, &dev, &error_fatal);
                 visit_free(v);
-                audio_define(dev);
-                select_soundhw(model, dev->id);
-                g_free(model);
+                if (model) {
+                    audio_define(dev);
+                    select_soundhw(model, dev->id);
+                    g_free(model);
+                } else {
+                    audio_define_default(dev, &error_fatal);
+                }
                 break;
             }
             case QEMU_OPTION_h: