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From: Peter Xu <peterx@redhat.com>
To: qemu-devel@nongnu.org
Cc: peterx@redhat.com, "Fabiano Rosas" <farosas@suse.de>,
	"Igor Mammedov" <imammedo@redhat.com>,
	"Juraj Marcin" <jmarcin@redhat.com>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	"Dr . David Alan Gilbert" <dave@treblig.org>,
	"Cédric Le Goater" <clg@redhat.com>,
	"Markus Armbruster" <armbru@redhat.com>,
	"Eduardo Habkost" <eduardo@habkost.net>,
	"Daniel P . Berrangé" <berrange@redhat.com>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Peter Maydell" <peter.maydell@linaro.org>
Subject: [PATCH 1/4] qom: TYPE_SINGLETON interface
Date: Thu, 24 Oct 2024 12:56:24 -0400	[thread overview]
Message-ID: <20241024165627.1372621-2-peterx@redhat.com> (raw)
In-Reply-To: <20241024165627.1372621-1-peterx@redhat.com>

Signed-off-by: Peter Xu <peterx@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



  reply	other threads:[~2024-10-24 16:57 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-10-24 16:56 [PATCH 0/4] QOM: Singleton interface Peter Xu
2024-10-24 16:56 ` Peter Xu [this message]
2024-10-24 20:02   ` [PATCH 1/4] qom: TYPE_SINGLETON interface Philippe Mathieu-Daudé
2024-10-24 20:53     ` Peter Xu
2024-10-25 15:11       ` Philippe Mathieu-Daudé
2024-10-25 16:21         ` Peter Xu
2024-10-25  8:07   ` Markus Armbruster
2024-10-25 15:17     ` Peter Xu
2024-10-25  9:51   ` Daniel P. Berrangé
2024-10-25 16:17     ` Peter Xu
2024-10-25 16:22       ` Daniel P. Berrangé
2024-10-25 22:10         ` Peter Xu
2024-10-29  0:01           ` Peter Xu
2024-10-25 16:37     ` Peter Xu
2024-10-24 16:56 ` [PATCH 2/4] x86/iommu: Make x86-iommu a singleton object Peter Xu
2024-10-25  9:25   ` Markus Armbruster
2024-10-25 21:55     ` Peter Xu
2024-10-25 22:13       ` Peter Xu
2024-11-07 11:12         ` Markus Armbruster
2024-11-07 15:29           ` Peter Xu
2024-11-08  8:50             ` Markus Armbruster
2024-10-29 10:47   ` Daniel P. Berrangé
2024-10-29 14:32     ` Peter Xu
2024-10-24 16:56 ` [PATCH 3/4] migration: Make migration object " Peter Xu
2024-10-24 19:20   ` Fabiano Rosas
2024-10-24 16:56 ` [PATCH 4/4] migration: Reset current_migration properly Peter Xu
2024-10-24 19:34   ` Fabiano Rosas
2024-10-24 20:15     ` Peter Xu
2024-10-24 20:51       ` Fabiano Rosas
2024-10-25  7:38 ` [PATCH 0/4] QOM: Singleton interface Markus Armbruster
2024-10-25 15:01   ` Peter Xu
2024-10-29 10:42     ` Daniel P. Berrangé
2024-10-29 14:45       ` Peter Xu
2024-10-29 16:04         ` Daniel P. Berrangé
2024-10-29 17:05           ` Peter Xu
2024-10-29 17:17             ` Daniel P. Berrangé
2024-12-11  8:19             ` Markus Armbruster
2024-12-11 22:10               ` Peter Xu

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