monitor: implement "user creatable" interface for adding monitors

Implement the user creatable QOM interface and define the monitor-qmp
and monitor-hmp types in QAPI. This unlocks the ability to create them
on the command line with -object or in HMP/QMP with object_add.

For example:

  $QEMU -chardev stdio,id=monchr0 -object monitor-hmp,id=mon0,chrdev=monchr0

Initially the "prepare_delete" callback is hardcoded to return an error
which means -object and object_add can be used, but object_del will fail.
Support for deleting monitors will be introduced in subsequent commits.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20260706135824.2623960-24-berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

monitor/hmp.c

@@ -40,6 +40,7 @@
 #include "qemu/target-info.h"
 #include "qemu/units.h"
 #include "qapi/error.h"
+#include "qom/object_interfaces.h"
 #include "exec/gdbstub.h"
 #include "system/block-backend.h"
 #include "trace.h"
@@ -71,10 +72,13 @@ static void monitor_hmp_set_readline(Object *obj, bool val, Error **errp)
 int monitor_hmp_vprintf(Monitor *mon, const char *fmt, va_list ap)
     G_GNUC_PRINTF(2, 0);
 static void monitor_hmp_accept_input(Monitor *mon);
+static void monitor_hmp_complete(UserCreatable *uc, Error **errp);
+static bool monitor_hmp_prepare_delete(UserCreatable *uc, Error **errp);
 
 static void monitor_hmp_class_init(ObjectClass *cls, const void *data)
 {
     MonitorClass *moncls = MONITOR_CLASS(cls);
+    UserCreatableClass *ucc = USER_CREATABLE_CLASS(cls);
 
     object_class_property_add_bool(cls, "readline",
                                    monitor_hmp_get_readline,
@@ -82,6 +86,9 @@ static void monitor_hmp_class_init(ObjectClass *cls, const void *data)
 
     moncls->vprintf = monitor_hmp_vprintf;
     moncls->accept_input = monitor_hmp_accept_input;
+
+    ucc->complete = monitor_hmp_complete;
+    ucc->prepare_delete = monitor_hmp_prepare_delete;
 }
 
 static void monitor_hmp_init(Object *obj)
@@ -1605,29 +1612,31 @@ static void monitor_readline_flush(void *opaque)
 void monitor_new_hmp(const char *id, const char *chardev_id,
                      bool use_readline, Error **errp)
 {
-    ERRP_GUARD();
-    MonitorHMP *mon;
     g_autofree char *autoid = id ? NULL : monitor_compat_id();
-    Object *obj = object_new_with_props(TYPE_MONITOR_HMP,
+    object_new_with_props(TYPE_MONITOR_HMP,
                           object_get_objects_root(),
                           id ? id : autoid,
                           errp,
                           "chardev", chardev_id,
                           "readline", use_readline ? "yes" : "no",
                           NULL);
+}
 
-    if (!obj) {
-        return;
-    }
+static void monitor_hmp_complete(UserCreatable *uc, Error **errp)
+{
+    MonitorHMP *mon = MONITOR_HMP(uc);
+    UserCreatableClass *ucc_parent =
+        USER_CREATABLE_CLASS(
+            object_class_get_parent(
+                OBJECT_CLASS(MONITOR_HMP_GET_CLASS(mon))));
+    ERRP_GUARD();
 
-    mon = MONITOR_HMP(obj);
-
-    monitor_complete(MONITOR(mon), errp);
+    ucc_parent->complete(uc, errp);
     if (*errp) {
-        object_unparent(OBJECT(mon));
         return;
     }
 
+    if (mon->parent_obj.chardev_id) {
         if (mon->use_readline) {
             mon->rs = readline_init(monitor_readline_printf,
                                     monitor_readline_flush,
@@ -1637,9 +1646,18 @@ void monitor_new_hmp(const char *id, const char *chardev_id,
         }
 
         qemu_chr_fe_set_handlers(&mon->parent_obj.chr,
-                             monitor_can_read, monitor_read, monitor_event,
-                             NULL, &mon->parent_obj, NULL, true);
+                                 monitor_can_read,
+                                 monitor_read,
+                                 monitor_event, NULL,
+                                 &mon->parent_obj, NULL, true);
         monitor_list_append(&mon->parent_obj);
+    }
+}
+
+static bool monitor_hmp_prepare_delete(UserCreatable *uc, Error **errp)
+{
+    error_setg(errp, "Deleting HMP monitors is not supported");
+    return false;
 }
 
 /**

monitor/monitor-internal.h

@@ -204,7 +204,6 @@ extern QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
 extern QemuMutex monitor_lock;
 extern MonitorList mon_list;
 
-void monitor_complete(Monitor *mon, Error **errp);
 bool monitor_requires_iothread(const Monitor *mon);
 int monitor_can_read(void *opaque);
 void monitor_list_append(Monitor *mon);

monitor/monitor.c

@@ -29,6 +29,7 @@
 #include "qapi/qapi-emit-events.h"
 #include "qapi/qapi-visit-control.h"
 #include "qobject/qdict.h"
+#include "qom/object_interfaces.h"
 #include "qemu/error-report.h"
 #include "qemu/option.h"
 #include "system/qtest.h"
@@ -75,7 +76,8 @@ static bool monitor_destroyed;
 
 int monitor_device_index;
 
-OBJECT_DEFINE_ABSTRACT_TYPE(Monitor, monitor, MONITOR, OBJECT);
+OBJECT_DEFINE_TYPE_EXTENDED(Monitor, monitor, MONITOR, OBJECT, true,
+                            { TYPE_USER_CREATABLE }, {});
 
 static void monitor_finalize(Object *obj)
 {
@@ -103,11 +105,17 @@ static void monitor_set_chardev_id(Object *obj, const char *str, Error **errp)
     mon->chardev_id = g_strdup(str);
 }
 
+static void monitor_complete(UserCreatable *uc, Error **errp);
+
 static void monitor_class_init(ObjectClass *cls, const void *data)
 {
+    UserCreatableClass *ucc = USER_CREATABLE_CLASS(cls);
+
     object_class_property_add_str(cls, "chardev",
                                   monitor_get_chardev_id,
                                   monitor_set_chardev_id);
+
+    ucc->complete = monitor_complete;
 }
 
 static void monitor_init(Object *obj)
@@ -689,8 +697,10 @@ char *monitor_compat_id(void)
     return g_strdup_printf("compat_monitor%d", monitor_device_index++);
 }
 
-void monitor_complete(Monitor *mon, Error **errp)
+static void monitor_complete(UserCreatable *uc, Error **errp)
 {
+    Monitor *mon = MONITOR(uc);
+
     if (mon->chardev_id) {
         Chardev *chr = qemu_chr_find(mon->chardev_id);
         if (chr == NULL) {

monitor/qmp.c

@@ -31,6 +31,7 @@
 #include "qobject/qdict.h"
 #include "qobject/qjson.h"
 #include "qobject/qlist.h"
+#include "qom/object_interfaces.h"
 #include "trace.h"
 
 /*
@@ -101,10 +102,13 @@ static void monitor_qmp_set_pretty(Object *obj, bool val, Error **errp)
 
 static void monitor_qmp_emit_event(Monitor *mon, QAPIEvent event, QDict *qdict);
 static bool monitor_qmp_requires_iothread(const Monitor *mon);
+static void monitor_qmp_complete(UserCreatable *uc, Error **errp);
+static bool monitor_qmp_prepare_delete(UserCreatable *uc, Error **errp);
 
 static void monitor_qmp_class_init(ObjectClass *cls, const void *data)
 {
     MonitorClass *moncls = MONITOR_CLASS(cls);
+    UserCreatableClass *ucc = USER_CREATABLE_CLASS(cls);
 
     object_class_property_add_bool(cls, "pretty",
                                    monitor_qmp_get_pretty,
@@ -112,6 +116,9 @@ static void monitor_qmp_class_init(ObjectClass *cls, const void *data)
 
     moncls->emit_event = monitor_qmp_emit_event;
     moncls->requires_iothread = monitor_qmp_requires_iothread;
+
+    ucc->complete = monitor_qmp_complete;
+    ucc->prepare_delete = monitor_qmp_prepare_delete;
 }
 
 static void handle_qmp_command(void *opaque, QObject *req, Error *err);
@@ -583,25 +590,27 @@ static void monitor_qmp_setup_handlers_bh(void *opaque)
 void monitor_new_qmp(const char *id, const char *chardev_id,
                      bool pretty, Error **errp)
 {
-    ERRP_GUARD();
-    MonitorQMP *mon;
     g_autofree char *autoid = id ? NULL : monitor_compat_id();
-    Object *obj = object_new_with_props(TYPE_MONITOR_QMP,
+    object_new_with_props(TYPE_MONITOR_QMP,
                           object_get_objects_root(),
                           id ? id : autoid,
                           errp,
                           "chardev", chardev_id,
                           "pretty", pretty ? "yes" : "no",
                           NULL);
+}
 
-    if (!obj) {
-        return;
-    }
+static void monitor_qmp_complete(UserCreatable *uc, Error **errp)
+{
+    MonitorQMP *mon = MONITOR_QMP(uc);
+    UserCreatableClass *ucc_parent =
+        USER_CREATABLE_CLASS(
+            object_class_get_parent(
+                OBJECT_CLASS(MONITOR_QMP_GET_CLASS(mon))));
+    ERRP_GUARD();
 
-    mon = MONITOR_QMP(obj);
-    monitor_complete(MONITOR(mon), errp);
+    ucc_parent->complete(uc, errp);
     if (*errp) {
-        object_unparent(OBJECT(mon));
         return;
     }
 
@@ -633,3 +642,9 @@ void monitor_new_qmp(const char *id, const char *chardev_id,
         monitor_list_append(&mon->parent_obj);
     }
 }
+
+static bool monitor_qmp_prepare_delete(UserCreatable *uc, Error **errp)
+{
+    error_setg(errp, "Deleting QMP monitors is not supported");
+    return false;
+}

qapi/qom.json

@@ -1187,6 +1187,45 @@
   'data': { '*cpu-affinity': ['uint16'],
             '*node-affinity': ['uint16'] } }
 
+##
+# @MonitorProperties:
+#
+# Properties for all monitors
+#
+# @chardev: ID of the character device providing the monitor transport
+#
+# Since: 11.1
+##
+{ 'struct': 'MonitorProperties',
+  'data': { 'chardev': 'str' }}
+
+##
+# @MonitorHMPProperties:
+#
+# Properties for the HMP monitor
+#
+# @readline: whether to enable readline for line editing
+#      (default: true)
+#
+# Since: 11.1
+##
+{ 'struct': 'MonitorHMPProperties',
+  'base': 'MonitorProperties',
+  'data': { '*readline': 'bool' } }
+
+##
+# @MonitorQMPProperties:
+#
+# Properties for the QMP monitor
+#
+# @pretty: whether to pretty print JSON responses (default: false)
+#
+# Since: 11.1
+##
+{ 'struct': 'MonitorQMPProperties',
+  'base': 'MonitorProperties',
+  'data': { '*pretty': 'bool' } }
+
 ##
 # @ObjectType:
 #
@@ -1237,6 +1276,8 @@
     'memory-backend-ram',
     { 'name': 'memory-backend-shm',
       'if': 'CONFIG_POSIX' },
+    'monitor-hmp',
+    'monitor-qmp',
     'pef-guest',
     { 'name': 'pr-manager-helper',
       'if': 'CONFIG_LINUX' },
@@ -1315,6 +1356,8 @@
       'memory-backend-ram':         'MemoryBackendProperties',
       'memory-backend-shm':         { 'type': 'MemoryBackendShmProperties',
                                       'if': 'CONFIG_POSIX' },
+      'monitor-hmp':                'MonitorHMPProperties',
+      'monitor-qmp':                'MonitorQMPProperties',
       'pr-manager-helper':          { 'type': 'PrManagerHelperProperties',
                                       'if': 'CONFIG_LINUX' },
       'qtest':                      'QtestProperties',

stubs/monitor-internal.c

@@ -11,4 +11,5 @@ int monitor_get_fd(Monitor *mon, const char *name, Error **errp)
 void monitor_new_hmp(const char *id, const char *chardev_id,
                      bool use_readline, Error **errp)
 {
+    g_assert_not_reached();
 }

system/vl.c

@@ -1829,6 +1829,10 @@ static void object_option_add_visitor(Visitor *v)
 {
     ObjectOption *opt = g_new0(ObjectOption, 1);
     visit_type_ObjectOptions(v, NULL, &opt->opts, &error_fatal);
+    if (opt->opts->qom_type == OBJECT_TYPE_MONITOR_HMP ||
+        opt->opts->qom_type == OBJECT_TYPE_MONITOR_QMP) {
+        default_monitor = 0;
+    }
     QTAILQ_INSERT_TAIL(&object_opts, opt, next);
 }
 
@@ -1970,7 +1974,9 @@ static bool object_create_early(const char *type)
 
     /* Reason: property "chardev" */
     if (g_str_equal(type, "rng-egd") ||
-        g_str_equal(type, "qtest")) {
+        g_str_equal(type, "qtest") ||
+        g_str_equal(type, "monitor-hmp") ||
+        g_str_equal(type, "monitor-qmp")) {
         return false;
     }