Re: [PATCH v2 5/9] virtio_net: construct multi-buffer xdp in mergeable

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在 2022/12/20 22:14, Heng Qi 写道:
Build multi-buffer xdp using virtnet_build_xdp_buff_mrg().

For the prefilled buffer before xdp is set, we will probably use
vq reset in the future. At the same time, virtio net currently
uses comp pages, and bpf_xdp_frags_increase_tail() needs to calculate
the tailroom of the last frag, which will involve the offset of the
corresponding page and cause a negative value, so we disable tail
increase by not setting xdp_rxq->frag_size.

Signed-off-by: Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>
Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
---
  drivers/net/virtio_net.c | 60 +++++++++++++++++++++++++++++-----------
  1 file changed, 44 insertions(+), 16 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 8fc3b1841d92..40bc58fa57f5 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1018,6 +1018,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
  					 unsigned int *xdp_xmit,
  					 struct virtnet_rq_stats *stats)
  {
+	unsigned int tailroom = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
  	struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
  	u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
  	struct page *page = virt_to_head_page(buf);
@@ -1048,11 +1049,14 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
  	rcu_read_lock();
  	xdp_prog = rcu_dereference(rq->xdp_prog);
  	if (xdp_prog) {
+		unsigned int xdp_frags_truesz = 0;
+		struct skb_shared_info *shinfo;
  		struct xdp_frame *xdpf;
  		struct page *xdp_page;
  		struct xdp_buff xdp;
  		void *data;
  		u32 act;
+		int i;
/* Transient failure which in theory could occur if
  		 * in-flight packets from before XDP was enabled reach
@@ -1061,19 +1065,23 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
  		if (unlikely(hdr->hdr.gso_type))
  			goto err_xdp;
- /* Buffers with headroom use PAGE_SIZE as alloc size,
-		 * see add_recvbuf_mergeable() + get_mergeable_buf_len()
+		/* Now XDP core assumes frag size is PAGE_SIZE, but buffers
+		 * with headroom may add hole in truesize, which
+		 * make their length exceed PAGE_SIZE. So we disabled the
+		 * hole mechanism for xdp. See add_recvbuf_mergeable().
  		 */
  		frame_sz = headroom ? PAGE_SIZE : truesize;
- /* This happens when rx buffer size is underestimated
-		 * or headroom is not enough because of the buffer
-		 * was refilled before XDP is set. This should only
-		 * happen for the first several packets, so we don't
-		 * care much about its performance.
+		/* This happens when headroom is not enough because
+		 * of the buffer was prefilled before XDP is set.
+		 * This should only happen for the first several packets.
+		 * In fact, vq reset can be used here to help us clean up
+		 * the prefilled buffers, but many existing devices do not
+		 * support it, and we don't want to bother users who are
+		 * using xdp normally.
  		 */
-		if (unlikely(num_buf > 1 ||
-			     headroom < virtnet_get_headroom(vi))) {
+		if (!xdp_prog->aux->xdp_has_frags &&
+		    (num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
  			/* linearize data for XDP */
  			xdp_page = xdp_linearize_page(rq, &num_buf,
  						      page, offset,
@@ -1084,17 +1092,26 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
  			if (!xdp_page)
  				goto err_xdp;
  			offset = VIRTIO_XDP_HEADROOM;
+		} else if (unlikely(headroom < virtnet_get_headroom(vi))) {


I believe we need to check xdp_prog->aux->xdp_has_frags at least since this may not work if it needs more than one frags?

Btw, I don't see a reason why we can't reuse xdp_linearize_page(), (we probably don't need error is the buffer exceeds PAGE_SIZE).

Other looks good.

Thanks


+			if ((VIRTIO_XDP_HEADROOM + len + tailroom) > PAGE_SIZE)
+				goto err_xdp;
+
+			xdp_page = alloc_page(GFP_ATOMIC);
+			if (!xdp_page)
+				goto err_xdp;
+
+			memcpy(page_address(xdp_page) + VIRTIO_XDP_HEADROOM,
+			       page_address(page) + offset, len);
+			frame_sz = PAGE_SIZE;
+			offset = VIRTIO_XDP_HEADROOM;
  		} else {
  			xdp_page = page;
  		}
-
-		/* Allow consuming headroom but reserve enough space to push
-		 * the descriptor on if we get an XDP_TX return code.
-		 */
  		data = page_address(xdp_page) + offset;
-		xdp_init_buff(&xdp, frame_sz - vi->hdr_len, &rq->xdp_rxq);
-		xdp_prepare_buff(&xdp, data - VIRTIO_XDP_HEADROOM + vi->hdr_len,
-				 VIRTIO_XDP_HEADROOM, len - vi->hdr_len, true);
+		err = virtnet_build_xdp_buff_mrg(dev, vi, rq, &xdp, data, len, frame_sz,
+						 &num_buf, &xdp_frags_truesz, stats);
+		if (unlikely(err))
+			goto err_xdp_frags;
act = bpf_prog_run_xdp(xdp_prog, &xdp);
  		stats->xdp_packets++;
@@ -1190,6 +1207,17 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
  				__free_pages(xdp_page, 0);
  			goto err_xdp;
  		}
+err_xdp_frags:
+		shinfo = xdp_get_shared_info_from_buff(&xdp);
+
+		if (unlikely(xdp_page != page))
+			__free_pages(xdp_page, 0);
+
+		for (i = 0; i < shinfo->nr_frags; i++) {
+			xdp_page = skb_frag_page(&shinfo->frags[i]);
+			put_page(xdp_page);
+		}
+		goto err_xdp;
  	}
  	rcu_read_unlock();




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