[RFC PATCH 7/9] virtio_net: build skb from multi-buffer xdp

[Heng Qi <hengqi@linux.alibaba.com>]

This converts a multi-buffer xdp_buff directly to a skb.

Now, we have isolated the construction of skb based on
multi-buffer xdp from page_to_skb().

Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/net/virtio_net.c | 50 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 83e6933ae62b..260ce722c687 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -911,6 +911,56 @@ static struct sk_buff *receive_big(struct net_device *dev,
 	return NULL;
 }
 
+/* Why not use xdp_build_skb_from_frame() ?
+ * XDP core assumes that xdp frags are PAGE_SIZE in length, while in
+ * virtio-net there are 2 points that do not match its requirements:
+ *  1. The size of the prefilled buffer is not fixed before xdp is set.
+ *  2. When xdp is loaded, virtio-net has a hole mechanism (refer to
+ *     add_recvbuf_mergeable()), which will make the size of a buffer
+ *     exceed PAGE_SIZE.
+ */
+static struct sk_buff *build_skb_from_xdp_buff(struct net_device *dev,
+					       struct virtnet_info *vi,
+					       struct xdp_buff *xdp,
+					       unsigned int xdp_frags_truesz)
+{
+	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
+	unsigned int headroom, data_len;
+	struct sk_buff *skb;
+	int metasize;
+	u8 nr_frags;
+
+	if (unlikely(xdp->data_end > xdp_data_hard_end(xdp))) {
+		pr_debug("Error building skb as missing reserved tailroom for xdp");
+		return NULL;
+	}
+
+	if (unlikely(xdp_buff_has_frags(xdp)))
+		nr_frags = sinfo->nr_frags;
+
+	skb = build_skb(xdp->data_hard_start, xdp->frame_sz);
+	if (unlikely(!skb))
+		return NULL;
+
+	headroom = xdp->data - xdp->data_hard_start;
+	data_len = xdp->data_end - xdp->data;
+	skb_reserve(skb, headroom);
+	__skb_put(skb, data_len);
+
+	metasize = xdp->data - xdp->data_meta;
+	metasize = metasize > 0 ? metasize : 0;
+	if (metasize)
+		skb_metadata_set(skb, metasize);
+
+	if (unlikely(xdp_buff_has_frags(xdp)))
+		xdp_update_skb_shared_info(skb, nr_frags,
+					   sinfo->xdp_frags_size,
+					   xdp_frags_truesz,
+					   xdp_buff_is_frag_pfmemalloc(xdp));
+
+	return skb;
+}
+
 static int virtnet_build_xdp_buff(struct net_device *dev,
 				  struct virtnet_info *vi,
 				  struct receive_queue *rq,
-- 
2.19.1.6.gb485710b