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

Heng Qi
2022-11-22T07:44:11Z

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