qmp: expose list of supported character device backends

Introduce 'query-chardev-backends' QMP command which lists all
supported character device backends.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qapi-schema.json

@@ -436,6 +436,28 @@
 ##
 { 'command': 'query-chardev', 'returns': ['ChardevInfo'] }
 
+##
+# @ChardevBackendInfo:
+#
+# Information about a character device backend
+#
+# @name: The backend name
+#
+# Since: 2.0
+##
+{ 'type': 'ChardevBackendInfo', 'data': {'name': 'str'} }
+
+##
+# @query-chardev-backends:
+#
+# Returns information about character device backends.
+#
+# Returns: a list of @ChardevBackendInfo
+#
+# Since: 2.0
+##
+{ 'command': 'query-chardev-backends', 'returns': ['ChardevBackendInfo'] }
+
 ##
 # @DataFormat:
 #

qemu-char.c

@@ -3432,6 +3432,25 @@ ChardevInfoList *qmp_query_chardev(Error **errp)
     return chr_list;
 }
 
+ChardevBackendInfoList *qmp_query_chardev_backends(Error **errp)
+{
+    ChardevBackendInfoList *backend_list = NULL;
+    CharDriver *c = NULL;
+    GSList *i = NULL;
+
+    for (i = backends; i; i = i->next) {
+        ChardevBackendInfoList *info = g_malloc0(sizeof(*info));
+        c = i->data;
+        info->value = g_malloc0(sizeof(*info->value));
+        info->value->name = g_strdup(c->name);
+
+        info->next = backend_list;
+        backend_list = info;
+    }
+
+    return backend_list;
+}
+
 CharDriverState *qemu_chr_find(const char *name)
 {
     CharDriverState *chr;

qmp-commands.hx

@@ -1923,6 +1923,47 @@ EQMP
         .mhandler.cmd_new = qmp_marshal_input_query_chardev,
     },
 
+SQMP
+query-chardev-backends
+-------------
+
+List available character device backends.
+
+Each backend is represented by a json-object, the returned value is a json-array
+of all backends.
+
+Each json-object contains:
+
+- "name": backend name (json-string)
+
+Example:
+
+-> { "execute": "query-chardev-backends" }
+<- {
+      "return":[
+         {
+            "name":"udp"
+         },
+         {
+            "name":"tcp"
+         },
+         {
+            "name":"unix"
+         },
+         {
+            "name":"spiceport"
+         }
+      ]
+   }
+
+EQMP
+
+    {
+        .name       = "query-chardev-backends",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_marshal_input_query_chardev_backends,
+    },
+
 SQMP
 query-block
 -----------