vdpa: introduce get_vq_notification method

This patch introduces a new method in the vdpa_config_ops which
reports the physical address and the size of the doorbell for a
specific virtqueue.

This will be used by the future patches that maps doorbell to
userspace.
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200529080303.15449-4-jasowang@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

include/linux/vdpa.h

@@ -17,6 +17,16 @@ struct vdpa_callback {
 	void *private;
 };
 
+/**
+ * vDPA notification area
+ * @addr: base address of the notification area
+ * @size: size of the notification area
+ */
+struct vdpa_notification_area {
+	resource_size_t addr;
+	resource_size_t size;
+};
+
 /**
  * vDPA device - representation of a vDPA device
  * @dev: underlying device
@@ -73,6 +83,10 @@ struct vdpa_device {
  *				@vdev: vdpa device
  *				@idx: virtqueue index
  *				Returns virtqueue state (last_avail_idx)
+ * @get_vq_notification: 	Get the notification area for a virtqueue
+ *				@vdev: vdpa device
+ *				@idx: virtqueue index
+ *				Returns the notifcation area
  * @get_vq_align:		Get the virtqueue align requirement
  *				for the device
  *				@vdev: vdpa device
@@ -162,6 +176,8 @@ struct vdpa_config_ops {
 	bool (*get_vq_ready)(struct vdpa_device *vdev, u16 idx);
 	int (*set_vq_state)(struct vdpa_device *vdev, u16 idx, u64 state);
 	u64 (*get_vq_state)(struct vdpa_device *vdev, u16 idx);
+	struct vdpa_notification_area
+	(*get_vq_notification)(struct vdpa_device *vdev, u16 idx);
 
 	/* Device ops */
 	u32 (*get_vq_align)(struct vdpa_device *vdev);