vdpa/mlx5: preserve CVQ vringh index

mlx5_vdpa does not preserve userland's view of vring base for the control
queue in the following sequence:

ioctl VHOST_SET_VRING_BASE
ioctl VHOST_VDPA_SET_STATUS VIRTIO_CONFIG_S_DRIVER_OK
  mlx5_vdpa_set_status()
    setup_cvq_vring()
      vringh_init_iotlb()
        vringh_init_kern()
          vrh->last_avail_idx = 0;
ioctl VHOST_GET_VRING_BASE

To fix, restore the value of cvq->vring.last_avail_idx after calling
vringh_init_iotlb.

Fixes: 5262912e ("vdpa/mlx5: Add support for control VQ and MAC setting")
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <1699014387-194368-1-git-send-email-steven.sistare@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/mlx5/net/mlx5_vnet.c

@@ -2815,13 +2815,18 @@ static int setup_cvq_vring(struct mlx5_vdpa_dev *mvdev)
 	struct mlx5_control_vq *cvq = &mvdev->cvq;
 	int err = 0;
 
-	if (mvdev->actual_features & BIT_ULL(VIRTIO_NET_F_CTRL_VQ))
+	if (mvdev->actual_features & BIT_ULL(VIRTIO_NET_F_CTRL_VQ)) {
+		u16 idx = cvq->vring.last_avail_idx;
+
 		err = vringh_init_iotlb(&cvq->vring, mvdev->actual_features,
 					MLX5_CVQ_MAX_ENT, false,
 					(struct vring_desc *)(uintptr_t)cvq->desc_addr,
 					(struct vring_avail *)(uintptr_t)cvq->driver_addr,
 					(struct vring_used *)(uintptr_t)cvq->device_addr);
 
+		if (!err)
+			cvq->vring.last_avail_idx = cvq->vring.last_used_idx = idx;
+	}
 	return err;
 }