Re: [net-next PATCH v4 5/6] virtio_net: add XDP_TX support

This adds support for the XDP_TX action to virtio_net. When an XDP
program is run and returns the XDP_TX action the virtio_net XDP
implementation will transmit the packet on a TX queue that aligns
with the current CPU that the XDP packet was processed on.

Before sending the packet the header is zeroed.  Also XDP is expected
to handle checksum correctly so no checksum offload  support is
provided.

Signed-off-by: John Fastabend <redacted>
---
 drivers/net/virtio_net.c |   63 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 60 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b67203e..137caba 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -330,12 +330,43 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 	return skb;
 }
 
+static void virtnet_xdp_xmit(struct virtnet_info *vi,
+			     unsigned int qnum, struct xdp_buff *xdp)
+{
+	struct send_queue *sq = &vi->sq[qnum];
+	struct virtio_net_hdr_mrg_rxbuf *hdr;
+	unsigned int num_sg, len;
+	void *xdp_sent;
+	int err;
+
+	/* Free up any pending old buffers before queueing new ones. */
+	while ((xdp_sent = virtqueue_get_buf(sq->vq, &len)) != NULL) {
+		struct page *page = virt_to_head_page(xdp_sent);
+
+		put_page(page);
+	}
+
+	/* Zero header and leave csum up to XDP layers */
+	hdr = xdp->data;
+	memset(hdr, 0, vi->hdr_len);
+
+	num_sg = 1;
+	sg_init_one(sq->sg, xdp->data, xdp->data_end - xdp->data);
+	err = virtqueue_add_outbuf(sq->vq, sq->sg, num_sg,
+				   xdp->data, GFP_ATOMIC);
+	if (unlikely(err))
+		put_page(virt_to_head_page(xdp->data));
+	else
+		virtqueue_kick(sq->vq);
+}
+