Signed-off-by: Peter Xu <pet...@redhat.com>
---
include/qom/object_interfaces.h | 47 +++++++++++++++++++++++++++++++++
qom/object.c | 3 +++
qom/object_interfaces.c | 24 +++++++++++++++++
qom/qom-qmp-cmds.c | 22 ++++++++++++---
system/qdev-monitor.c | 7 +++++
5 files changed, 100 insertions(+), 3 deletions(-)
diff --git a/include/qom/object_interfaces.h b/include/qom/object_interfaces.h
index 02b11a7ef0..9b2cc0e554 100644
--- a/include/qom/object_interfaces.h
+++ b/include/qom/object_interfaces.h
@@ -177,4 +177,51 @@ bool user_creatable_del(const char *id, Error **errp);
*/
void user_creatable_cleanup(void);
+#define TYPE_SINGLETON "singleton"
+
+typedef struct SingletonClass SingletonClass;
+DECLARE_CLASS_CHECKERS(SingletonClass, SINGLETON, TYPE_SINGLETON)
+
+/**
+ * SingletonClass:
+ *
+ * @parent_class: the base class
+ * @get_instance: fetch the singleton instance if it is created,
+ * NULL otherwise.
+ *
+ * Singleton class describes the type of object classes that can only
+ * provide one instance for the whole lifecycle of QEMU. It will fail the
+ * operation if one attemps to create more than one instance.
+ *
+ * One can fetch the single object using class's get_instance() callback if
+ * it was created before. This can be useful for operations like QMP
+ * qom-list-properties, where dynamically creating an object might not be
+ * feasible.
+ */
+struct SingletonClass {
+ /* <private> */
+ InterfaceClass parent_class;
+ /* <public> */
+ Object *(*get_instance)(Error **errp);
+};
+
+/**
+ * object_class_is_singleton:
+ *
+ * @class: the class to detect singleton
+ *
+ * Returns: true if it's a singleton class, false otherwise.
+ */
+bool object_class_is_singleton(ObjectClass *class);
+
+/**
+ * singleton_get_instance:
+ *
+ * @class: the class to fetch singleton instance
+ *
+ * Returns: the object* if the class is a singleton class and the singleton
+ * object is created, NULL otherwise.
+ */
+Object *singleton_get_instance(ObjectClass *class);
+
#endif
diff --git a/qom/object.c b/qom/object.c
index 11424cf471..ded299ae1a 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -553,6 +553,9 @@ static void object_initialize_with_type(Object *obj, size_t
size, TypeImpl *type
g_assert(type->abstract == false);
g_assert(size >= type->instance_size);
+ /* Singleton class can only create one object */
+ g_assert(!singleton_get_instance(type->class));
+
memset(obj, 0, type->instance_size);
obj->class = type->class;
object_ref(obj);
diff --git a/qom/object_interfaces.c b/qom/object_interfaces.c
index e0833c8bfe..6766060d0a 100644
--- a/qom/object_interfaces.c
+++ b/qom/object_interfaces.c
@@ -354,6 +354,23 @@ void user_creatable_cleanup(void)
object_unparent(object_get_objects_root());
}
+bool object_class_is_singleton(ObjectClass *class)
+{
+ return !!object_class_dynamic_cast(class, TYPE_SINGLETON);
+}
+
+Object *singleton_get_instance(ObjectClass *class)
+{
+ SingletonClass *singleton =
+ (SingletonClass *)object_class_dynamic_cast(class, TYPE_SINGLETON);
+
+ if (!singleton) {
+ return NULL;
+ }
+
+ return singleton->get_instance(&error_abort);
+}
+
static void register_types(void)
{
static const TypeInfo uc_interface_info = {
@@ -362,7 +379,14 @@ static void register_types(void)
.class_size = sizeof(UserCreatableClass),
};
+ static const TypeInfo singleton_interface_info = {
+ .name = TYPE_SINGLETON,
+ .parent = TYPE_INTERFACE,
+ .class_size = sizeof(SingletonClass),
+ };
+
type_register_static(&uc_interface_info);
+ type_register_static(&singleton_interface_info);
}
type_init(register_types)
diff --git a/qom/qom-qmp-cmds.c b/qom/qom-qmp-cmds.c
index e91a235347..ecc1cf781c 100644
--- a/qom/qom-qmp-cmds.c
+++ b/qom/qom-qmp-cmds.c
@@ -126,6 +126,7 @@ ObjectPropertyInfoList *qmp_device_list_properties(const
char *typename,
ObjectProperty *prop;
ObjectPropertyIterator iter;
ObjectPropertyInfoList *prop_list = NULL;
+ bool create;
klass = module_object_class_by_name(typename);
if (klass == NULL) {
@@ -141,7 +142,15 @@ ObjectPropertyInfoList *qmp_device_list_properties(const
char *typename,
return NULL;
}
- obj = object_new(typename);
+ /* Avoid creating multiple instances if the class is a singleton */
+ create = !object_class_is_singleton(klass) ||
+ !singleton_get_instance(klass);
+
+ if (create) {
+ obj = object_new(typename);
+ } else {
+ obj = singleton_get_instance(klass);
+ }
object_property_iter_init(&iter, obj);
while ((prop = object_property_iter_next(&iter))) {
@@ -172,7 +181,9 @@ ObjectPropertyInfoList *qmp_device_list_properties(const
char *typename,
QAPI_LIST_PREPEND(prop_list, info);
}
- object_unref(obj);
+ if (create) {
+ object_unref(obj);
+ }
return prop_list;
}
@@ -199,7 +210,12 @@ ObjectPropertyInfoList *qmp_qom_list_properties(const char
*typename,
return NULL;
}
- if (object_class_is_abstract(klass)) {
+ /*
+ * Abstract classes are not for instantiations, meanwhile avoid
+ * creating temporary singleton objects because it can cause conflicts
+ * if there's already one created.
+ */
+ if (object_class_is_abstract(klass) || object_class_is_singleton(klass)) {
object_class_property_iter_init(&iter, klass);
} else {
obj = object_new(typename);
diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
index 44994ea0e1..1310f35c9f 100644
--- a/system/qdev-monitor.c
+++ b/system/qdev-monitor.c
@@ -36,6 +36,7 @@
#include "qemu/option.h"
#include "qemu/qemu-print.h"
#include "qemu/option_int.h"
+#include "qom/object_interfaces.h"
#include "sysemu/block-backend.h"
#include "migration/misc.h"
#include "qemu/cutils.h"
@@ -643,6 +644,12 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
return NULL;
}
+ if (singleton_get_instance(OBJECT_CLASS(dc))) {
+ error_setg(errp, "Class '%s' only supports one instance",
+ driver);
+ return NULL;
+ }
+
/* find bus */
path = qdict_get_try_str(opts, "bus");
if (path != NULL) {
--
2.45.0
