qom: add a reference count to qdev objects

To ensure that a device isn't removed from the graph until all of its links are
broken.
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/qdev.c

@@ -323,6 +323,11 @@ int qdev_unplug(DeviceState *dev)
     }
     assert(dev->info->unplug != NULL);
 
+    if (dev->ref != 0) {
+        qerror_report(QERR_DEVICE_IN_USE, dev->id?:"");
+        return -1;
+    }
+
     qdev_hot_removed = true;
 
     return dev->info->unplug(dev);
@@ -962,3 +967,14 @@ char* qdev_get_fw_dev_path(DeviceState *dev)
 
     return strdup(path);
 }
+
+void qdev_ref(DeviceState *dev)
+{
+    dev->ref++;
+}
+
+void qdev_unref(DeviceState *dev)
+{
+    g_assert(dev->ref > 0);
+    dev->ref--;
+}

hw/qdev.h

@@ -45,6 +45,12 @@ struct DeviceState {
     QTAILQ_ENTRY(DeviceState) sibling;
     int instance_id_alias;
     int alias_required_for_version;
+
+    /**
+     * This tracks the number of references between devices.  See @qdev_ref for
+     * more information.
+     */
+    uint32_t ref;
 };
 
 typedef void (*bus_dev_printfn)(Monitor *mon, DeviceState *dev, int indent);
@@ -329,4 +335,24 @@ char *qdev_get_fw_dev_path(DeviceState *dev);
 /* This is a nasty hack to allow passing a NULL bus to qdev_create.  */
 extern struct BusInfo system_bus_info;
 
+/**
+ * @qdev_ref
+ *
+ * Increase the reference count of a device.  A device cannot be freed as long
+ * as its reference count is greater than zero.
+ *
+ * @dev - the device
+ */
+void qdev_ref(DeviceState *dev);
+
+/**
+ * @qdef_unref
+ *
+ * Decrease the reference count of a device.  A device cannot be freed as long
+ * as its reference count is greater than zero.
+ *
+ * @dev - the device
+ */
+void qdev_unref(DeviceState *dev);
+
 #endif