vdpa/mlx5: fix up endian-ness for mtu

VDPA mlx5 accesses config space as native endian - this is
wrong since it's a modern device and actually uses LE.

It only supports modern guests so we could punt and
just force LE, but let's use the full virtio APIs since people
tend to copy/paste code, and this is not data path anyway.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/mlx5/net/mlx5_vnet.c

@@ -137,6 +137,7 @@ struct mlx5_vdpa_net {
 	struct mlx5_fc *rx_counter;
 	struct mlx5_flow_handle *rx_rule;
 	bool setup;
+	u16 mtu;
 };
 
 static void free_resources(struct mlx5_vdpa_net *ndev);
@@ -1506,6 +1507,13 @@ static void teardown_virtqueues(struct mlx5_vdpa_net *ndev)
 	}
 }
 
+/* TODO: cross-endian support */
+static inline bool mlx5_vdpa_is_little_endian(struct mlx5_vdpa_dev *mvdev)
+{
+	return virtio_legacy_is_little_endian() ||
+		(mvdev->actual_features & (1ULL << VIRTIO_F_VERSION_1));
+}
+
 static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
 {
 	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
@@ -1519,6 +1527,8 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
 		return err;
 
 	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
+	ndev->config.mtu = __cpu_to_virtio16(mlx5_vdpa_is_little_endian(mvdev),
+					     ndev->mtu);
 	return err;
 }
 
@@ -1925,7 +1935,7 @@ void *mlx5_vdpa_add_dev(struct mlx5_core_dev *mdev)
 	init_mvqs(ndev);
 	mutex_init(&ndev->reslock);
 	config = &ndev->config;
-	err = mlx5_query_nic_vport_mtu(mdev, &config->mtu);
+	err = mlx5_query_nic_vport_mtu(mdev, &ndev->mtu);
 	if (err)
 		goto err_mtu;