virtio_net: introduce ability to get reply info from device

As the spec https://github.com/oasis-tcs/virtio-spec/commit/42f389989823039724f95bbbd243291ab0064f82

Based on the description provided in the above specification, we have
enabled the virtio-net driver to support acquiring some response
information from the device via the CVQ (Control Virtqueue).
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/virtio_net.c

@@ -2527,11 +2527,12 @@ static int virtnet_tx_resize(struct virtnet_info *vi,
  * supported by the hypervisor, as indicated by feature bits, should
  * never fail unless improperly formatted.
  */
-static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
-				 struct scatterlist *out)
+static bool virtnet_send_command_reply(struct virtnet_info *vi, u8 class, u8 cmd,
+				       struct scatterlist *out,
+				       struct scatterlist *in)
 {
-	struct scatterlist *sgs[4], hdr, stat;
-	unsigned out_num = 0, tmp;
+	struct scatterlist *sgs[5], hdr, stat;
+	u32 out_num = 0, tmp, in_num = 0;
 	int ret;
 
 	/* Caller should know better */
@@ -2549,10 +2550,13 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
 
 	/* Add return status. */
 	sg_init_one(&stat, &vi->ctrl->status, sizeof(vi->ctrl->status));
-	sgs[out_num] = &stat;
+	sgs[out_num + in_num++] = &stat;
 
-	BUG_ON(out_num + 1 > ARRAY_SIZE(sgs));
-	ret = virtqueue_add_sgs(vi->cvq, sgs, out_num, 1, vi, GFP_ATOMIC);
+	if (in)
+		sgs[out_num + in_num++] = in;
+
+	BUG_ON(out_num + in_num > ARRAY_SIZE(sgs));
+	ret = virtqueue_add_sgs(vi->cvq, sgs, out_num, in_num, vi, GFP_ATOMIC);
 	if (ret < 0) {
 		dev_warn(&vi->vdev->dev,
 			 "Failed to add sgs for command vq: %d\n.", ret);
@@ -2574,6 +2578,12 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
 	return vi->ctrl->status == VIRTIO_NET_OK;
 }
 
+static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
+				 struct scatterlist *out)
+{
+	return virtnet_send_command_reply(vi, class, cmd, out, NULL);
+}
+
 static int virtnet_set_mac_address(struct net_device *dev, void *p)
 {
 	struct virtnet_info *vi = netdev_priv(dev);