virtio_vdpa: setup correct vq size with callbacks get_vq_num_{max,min}

For the devices which implement the get_vq_num_min callback, the driver
should not negotiate with virtqueue size with the backend vdpa device if
the value returned by get_vq_num_min equals to the value returned by
get_vq_num_max.
This is useful for vdpa devices based on legacy virtio specfication.
Signed-off-by: Wu Zongyong <wuzongyong@linux.alibaba.com>
Link: https://lore.kernel.org/r/bc0551cec6c3f3dd9424b678b7c22d882aebab3a.1635493219.git.wuzongyong@linux.alibaba.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_vdpa.c

@@ -145,7 +145,8 @@ virtio_vdpa_setup_vq(struct virtio_device *vdev, unsigned int index,
 	/* Assume split virtqueue, switch to packed if necessary */
 	struct vdpa_vq_state state = {0};
 	unsigned long flags;
-	u32 align, num;
+	u32 align, max_num, min_num = 1;
+	bool may_reduce_num = true;
 	int err;
 
 	if (!name)
@@ -163,16 +164,21 @@ virtio_vdpa_setup_vq(struct virtio_device *vdev, unsigned int index,
 	if (!info)
 		return ERR_PTR(-ENOMEM);
 
-	num = ops->get_vq_num_max(vdpa);
-	if (num == 0) {
+	max_num = ops->get_vq_num_max(vdpa);
+	if (max_num == 0) {
 		err = -ENOENT;
 		goto error_new_virtqueue;
 	}
 
+	if (ops->get_vq_num_min)
+		min_num = ops->get_vq_num_min(vdpa);
+
+	may_reduce_num = (max_num == min_num) ? false : true;
+
 	/* Create the vring */
 	align = ops->get_vq_align(vdpa);
-	vq = vring_create_virtqueue(index, num, align, vdev,
-				    true, true, ctx,
+	vq = vring_create_virtqueue(index, max_num, align, vdev,
+				    true, may_reduce_num, ctx,
 				    virtio_vdpa_notify, callback, name);
 	if (!vq) {
 		err = -ENOMEM;