[Qemu-devel] [RFC 1/5] qdev: add child alias properties

[Stefan Hajnoczi <stefanha@redhat.com>]

Child alias properties allow a parent object to expose child object
properties.  This makes it possible for a parent object to forward all
or a subset of a child's properties.

Currently we achieve similar behavior by duplicating property
definitions and copying the fields around.  It turns out virtio has
several double-frees since we didn't get the memory management right.

Using child alias properties it will be much easier for a parent object
to set a child's properties without fragile memory management issues
since the actual field only exists once in the child object.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/core/qdev-properties.c    | 28 ++++++++++++++++++++++++++++
 include/hw/qdev-properties.h | 28 ++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/hw/core/qdev-properties.c b/hw/core/qdev-properties.c
index 77d0c66..e62a4fd 100644
--- a/hw/core/qdev-properties.c
+++ b/hw/core/qdev-properties.c
@@ -1002,3 +1002,31 @@ PropertyInfo qdev_prop_size = {
     .get = get_size,
     .set = set_size,
 };
+
+/* --- alias that forwards to a child object's property --- */
+
+static void get_child_alias(Object *obj, Visitor *v, void *opaque,
+                            const char *name, Error **errp)
+{
+    DeviceState *dev = DEVICE(obj);
+    Property *prop = opaque;
+    Object *child = OBJECT(qdev_get_prop_ptr(dev, prop));
+
+    object_property_get(child, v, name, errp);
+}
+
+static void set_child_alias(Object *obj, Visitor *v, void *opaque,
+                            const char *name, Error **errp)
+{
+    DeviceState *dev = DEVICE(obj);
+    Property *prop = opaque;
+    Object *child = OBJECT(qdev_get_prop_ptr(dev, prop));
+
+    object_property_set(child, v, name, errp);
+}
+
+PropertyInfo qdev_prop_child_alias = {
+    .name  = "ChildAlias",
+    .get = get_child_alias,
+    .set = set_child_alias,
+};
diff --git a/include/hw/qdev-properties.h b/include/hw/qdev-properties.h
index 3c000ee..5ab1cac 100644
--- a/include/hw/qdev-properties.h
+++ b/include/hw/qdev-properties.h
@@ -13,6 +13,7 @@ extern PropertyInfo qdev_prop_uint32;
 extern PropertyInfo qdev_prop_int32;
 extern PropertyInfo qdev_prop_uint64;
 extern PropertyInfo qdev_prop_size;
+extern PropertyInfo qdev_prop_child_alias;
 extern PropertyInfo qdev_prop_string;
 extern PropertyInfo qdev_prop_chr;
 extern PropertyInfo qdev_prop_ptr;
@@ -61,6 +62,33 @@ extern PropertyInfo qdev_prop_arraylen;
         .defval    = (bool)_defval,                              \
         }
 
+/**
+ * DEFINE_PROP_CHILD_ALIAS:
+ * @_name: property name
+ * @_state: name of the device state structure type
+ * @_field: name of field in @_state, must be Object subclass
+ *
+ * Define device properties that alias a child object's property.  For example,
+ * use the following to forward the 'baz' property where Foo embeds a Bar
+ * object:
+ *
+ *   typedef struct {
+ *       Object parent_obj;
+ *
+ *       Bar bar_obj;
+ *   } Foo;
+ *
+ *   DEFINE_PROP_CHILD_ALIAS("baz", Foo, bar_obj)
+ *
+ * Any access to Foo's 'baz' property actually accesses bar_obj's 'baz'
+ * property.
+ */
+#define DEFINE_PROP_CHILD_ALIAS(_name, _state, _field) {         \
+        .name      = (_name),                                    \
+        .info      = &(qdev_prop_child_alias),                   \
+        .offset    = offsetof(_state, _field),                   \
+        }
+
 #define PROP_ARRAY_LEN_PREFIX "len-"
 
 /**
-- 
1.8.5.3