vdpa/mlx5: should keep avail_index despite device status

A VM with mlx5 vDPA has below warnings while being reset:

vhost VQ 0 ring restore failed: -1: Resource temporarily unavailable (11)
vhost VQ 1 ring restore failed: -1: Resource temporarily unavailable (11)

We should allow userspace emulating the virtio device be
able to get to vq's avail_index, regardless of vDPA device
status. Save the index that was last seen when virtq was
stopped, so that userspace doesn't complain.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Link: https://lore.kernel.org/r/1601583511-15138-1-git-send-email-si-wei.liu@oracle.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eli Cohen <elic@nvidia.com>

drivers/vdpa/mlx5/net/mlx5_vnet.c

@@ -1133,15 +1133,17 @@ static void suspend_vq(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *m
 	if (!mvq->initialized)
 		return;
 
-	if (query_virtqueue(ndev, mvq, &attr)) {
-		mlx5_vdpa_warn(&ndev->mvdev, "failed to query virtqueue\n");
-		return;
-	}
 	if (mvq->fw_state != MLX5_VIRTIO_NET_Q_OBJECT_STATE_RDY)
 		return;
 
 	if (modify_virtqueue(ndev, mvq, MLX5_VIRTIO_NET_Q_OBJECT_STATE_SUSPEND))
 		mlx5_vdpa_warn(&ndev->mvdev, "modify to suspend failed\n");
+
+	if (query_virtqueue(ndev, mvq, &attr)) {
+		mlx5_vdpa_warn(&ndev->mvdev, "failed to query virtqueue\n");
+		return;
+	}
+	mvq->avail_idx = attr.available_index;
 }
 
 static void suspend_vqs(struct mlx5_vdpa_net *ndev)
@@ -1411,8 +1413,14 @@ static int mlx5_vdpa_get_vq_state(struct vdpa_device *vdev, u16 idx, struct vdpa
 	struct mlx5_virtq_attr attr;
 	int err;
 
-	if (!mvq->initialized)
-		return -EAGAIN;
+	/* If the virtq object was destroyed, use the value saved at
+	 * the last minute of suspend_vq. This caters for userspace
+	 * that cares about emulating the index after vq is stopped.
+	 */
+	if (!mvq->initialized) {
+		state->avail_index = mvq->avail_idx;
+		return 0;
+	}
 
 	err = query_virtqueue(ndev, mvq, &attr);
 	if (err) {