virtio: allow driver to disable the configure change notification

Sometime, it would be useful to disable the configure change
notification from the driver. So this patch allows this by introducing
a variable config_change_driver_disabled and only allow the configure
change notification callback to be triggered when it is allowed by
both the virtio core and the driver. It is set to false by default to
hold the current semantic so we don't need to change any drivers.

The first user for this would be virtio-net.

Cc: Venkat Venkatsubra <[e-mail]>
Cc: Gia-Khanh Nguyen <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Jason Wang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/virtio/virtio.c

···
 {
 	struct virtio_driver *drv = drv_to_virtio(dev->dev.driver);
 
-	if (!dev->config_core_enabled)
+	if (!dev->config_core_enabled || dev->config_driver_disabled)
 		dev->config_change_pending = true;
-	else if (drv && drv->config_changed)
+	else if (drv && drv->config_changed) {
 		drv->config_changed(dev);
+		dev->config_change_pending = false;
+	}
 }
 
 void virtio_config_changed(struct virtio_device *dev)
···
 	spin_unlock_irqrestore(&dev->config_lock, flags);
 }
 EXPORT_SYMBOL_GPL(virtio_config_changed);
+
+/**
+ * virtio_config_driver_disable - disable config change reporting by drivers
+ * @dev: the device to reset
+ *
+ * This is only allowed to be called by a driver and disabling can't
+ * be nested.
+ */
+void virtio_config_driver_disable(struct virtio_device *dev)
+{
+	spin_lock_irq(&dev->config_lock);
+	dev->config_driver_disabled = true;
+	spin_unlock_irq(&dev->config_lock);
+}
+EXPORT_SYMBOL_GPL(virtio_config_driver_disable);
+
+/**
+ * virtio_config_driver_enable - enable config change reporting by drivers
+ * @dev: the device to reset
+ *
+ * This is only allowed to be called by a driver and enabling can't
+ * be nested.
+ */
+void virtio_config_driver_enable(struct virtio_device *dev)
+{
+	spin_lock_irq(&dev->config_lock);
+	dev->config_driver_disabled = false;
+	if (dev->config_change_pending)
+		__virtio_config_changed(dev);
+	spin_unlock_irq(&dev->config_lock);
+}
+EXPORT_SYMBOL_GPL(virtio_config_driver_enable);
 
 static void virtio_config_core_disable(struct virtio_device *dev)
 {
···
 	dev->config_core_enabled = true;
 	if (dev->config_change_pending)
 		__virtio_config_changed(dev);
-	dev->config_change_pending = false;
 	spin_unlock_irq(&dev->config_lock);
 }
 

include/linux/virtio.h

···
  * @index: unique position on the virtio bus
  * @failed: saved value for VIRTIO_CONFIG_S_FAILED bit (for restore)
  * @config_core_enabled: configuration change reporting enabled by core
+ * @config_driver_disabled: configuration change reporting disabled by
+ *                          a driver
  * @config_change_pending: configuration change reported while disabled
  * @config_lock: protects configuration change reporting
  * @vqs_list_lock: protects @vqs.
···
 	int index;
 	bool failed;
 	bool config_core_enabled;
+	bool config_driver_disabled;
 	bool config_change_pending;
 	spinlock_t config_lock;
 	spinlock_t vqs_list_lock;
···
 void __virtqueue_unbreak(struct virtqueue *_vq);
 
 void virtio_config_changed(struct virtio_device *dev);
+
+void virtio_config_driver_disable(struct virtio_device *dev);
+void virtio_config_driver_enable(struct virtio_device *dev);
+
 #ifdef CONFIG_PM_SLEEP
 int virtio_device_freeze(struct virtio_device *dev);
 int virtio_device_restore(struct virtio_device *dev);