Re: [PATCH bpf-next v4] virtio_net: add XDP meta data support

On 2/5/20 1:10 PM, Jason Wang wrote:

On 2020/2/4 下午3:16, Yuya Kusakabe wrote:

quoted

Implement support for transferring XDP meta data into skb for
virtio_net driver; before calling into the program, xdp.data_meta points
to xdp.data and copy vnet header to the front of xdp.data_hard_start
to avoid overwriting it, where on program return with pass verdict,
we call into skb_metadata_set().

Fixes: de8f3a83b0a0 ("bpf: add meta pointer for direct access")
Signed-off-by: Yuya Kusakabe <redacted>
---
  drivers/net/virtio_net.c | 47 ++++++++++++++++++++++++++++------------
  1 file changed, 33 insertions(+), 14 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 2fe7a3188282..5fdd6ea0e3f1 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -371,7 +371,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,

                     struct receive_queue *rq,
                     struct page *page, unsigned int offset,
                     unsigned int len, unsigned int truesize,
-                   bool hdr_valid)
+                   bool hdr_valid, unsigned int metasize)
  {
      struct sk_buff *skb;
      struct virtio_net_hdr_mrg_rxbuf *hdr;

@@ -393,7 +393,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,

      else
          hdr_padded_len = sizeof(struct padded_vnet_hdr);
  -    if (hdr_valid)
+    if (hdr_valid && !metasize)

hdr_valid means no XDP, so I think we can remove the check for metasize here and add a comment instead?

I will fix it on next patch.

quoted

          memcpy(hdr, p, hdr_len);
        len -= hdr_len;

@@ -405,6 +405,11 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,

          copy = skb_tailroom(skb);
      skb_put_data(skb, p, copy);
  +    if (metasize) {
+        __skb_pull(skb, metasize);
+        skb_metadata_set(skb, metasize);
+    }
+
      len -= copy;
      offset += copy;
  @@ -644,6 +649,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
      unsigned int delta = 0;
      struct page *xdp_page;
      int err;
+    unsigned int metasize = 0;
        len -= vi->hdr_len;
      stats->bytes += len;

@@ -683,10 +689,15 @@ static struct sk_buff *receive_small(struct net_device *dev,

            xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
          xdp.data = xdp.data_hard_start + xdp_headroom;
-        xdp_set_data_meta_invalid(&xdp);
          xdp.data_end = xdp.data + len;
+        xdp.data_meta = xdp.data;
          xdp.rxq = &rq->xdp_rxq;
          orig_data = xdp.data;
+        /* Copy the vnet header to the front of data_hard_start to avoid
+         * overwriting it by XDP meta data.
+         */
+        memcpy(xdp.data_hard_start - vi->hdr_len,
+               xdp.data - vi->hdr_len, vi->hdr_len);

I think we don't need this. And it looks to me there's a bug in the current code.

Commit 436c9453a1ac0 ("virtio-net: keep vnet header zeroed after processing XDP") leave the a corner case for receive_small() which still use:

        if (!delta) {
                buf += header_offset;
                memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
        } /* keep zeroed vnet hdr since packet was changed by bpf */

Which seems wrong, we need check xdp_prog instead of delta.

With this fixed, there's no need to care about the vnet header here since we don't know whether or not packet is modified by XDP.

I missed this commit. I understand this is the reason for "Awaiting Upstream".

quoted

          act = bpf_prog_run_xdp(xdp_prog, &xdp);
          stats->xdp_packets++;
  @@ -695,9 +706,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
              /* Recalculate length in case bpf program changed it */
              delta = orig_data - xdp.data;
              len = xdp.data_end - xdp.data;
+            metasize = xdp.data - xdp.data_meta;
              break;
          case XDP_TX:
              stats->xdp_tx++;
+            xdp.data_meta = xdp.data;

I think we should remove the xdp_set_data_meta_invalid() at least? And move this initialization just after xdp.data is initialized.

Testing receive_small() requires to disable mrg_rxbuf, guest_tso4, guest_tso6 and guest_ufo from qemu command line.

quoted

              xdpf = convert_to_xdp_frame(&xdp);
              if (unlikely(!xdpf))
                  goto err_xdp;

@@ -736,10 +749,12 @@ static struct sk_buff *receive_small(struct net_device *dev,

      skb_reserve(skb, headroom - delta);
      skb_put(skb, len);
      if (!delta) {

Need to check xdp_prog (need another patch).

I will fix it on next patch.

quoted

-        buf += header_offset;
-        memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
+        memcpy(skb_vnet_hdr(skb), buf + VIRTNET_RX_PAD, vi->hdr_len);
      } /* keep zeroed vnet hdr since packet was changed by bpf */
  +    if (metasize)
+        skb_metadata_set(skb, metasize);
+
  err:
      return skb;
  @@ -760,8 +775,8 @@ static struct sk_buff *receive_big(struct net_device *dev,
                     struct virtnet_rq_stats *stats)
  {
      struct page *page = buf;
-    struct sk_buff *skb = page_to_skb(vi, rq, page, 0, len,
-                      PAGE_SIZE, true);
+    struct sk_buff *skb =
+        page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, true, 0);
        stats->bytes += len - vi->hdr_len;
      if (unlikely(!skb))

@@ -793,6 +808,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

      unsigned int truesize;
      unsigned int headroom = mergeable_ctx_to_headroom(ctx);
      int err;
+    unsigned int metasize = 0;
        head_skb = NULL;
      stats->bytes += len - vi->hdr_len;

@@ -839,8 +855,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

          data = page_address(xdp_page) + offset;
          xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
          xdp.data = data + vi->hdr_len;
-        xdp_set_data_meta_invalid(&xdp);
          xdp.data_end = xdp.data + (len - vi->hdr_len);
+        xdp.data_meta = xdp.data;
          xdp.rxq = &rq->xdp_rxq;
            act = bpf_prog_run_xdp(xdp_prog, &xdp);

@@ -852,8 +868,9 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

               * adjustments. Note other cases do not build an
               * skb and avoid using offset
               */
-            offset = xdp.data -
-                    page_address(xdp_page) - vi->hdr_len;
+            metasize = xdp.data - xdp.data_meta;
+            offset = xdp.data - page_address(xdp_page) -
+                 vi->hdr_len - metasize;
                /* recalculate len if xdp.data or xdp.data_end were
               * adjusted

@@ -863,14 +880,15 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

              if (unlikely(xdp_page != page)) {
                  rcu_read_unlock();
                  put_page(page);
-                head_skb = page_to_skb(vi, rq, xdp_page,
-                               offset, len,
-                               PAGE_SIZE, false);
+                head_skb = page_to_skb(vi, rq, xdp_page, offset,
+                               len, PAGE_SIZE, false,
+                               metasize);
                  return head_skb;
              }
              break;
          case XDP_TX:
              stats->xdp_tx++;
+            xdp.data_meta = xdp.data;

Any reason for doing this?

XDP_TX can not support metadata for now, because if metasize > 0, __virtnet_xdp_xmit_one() returns EOPNOTSUPP.

static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
				   struct send_queue *sq,
				   struct xdp_frame *xdpf)
{
	struct virtio_net_hdr_mrg_rxbuf *hdr;
	int err;

	/* virtqueue want to use data area in-front of packet */
	if (unlikely(xdpf->metasize > 0))
		return -EOPNOTSUPP;

Thanks

quoted

              xdpf = convert_to_xdp_frame(&xdp);
              if (unlikely(!xdpf))
                  goto err_xdp;

@@ -921,7 +939,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,

          goto err_skb;
      }
  -    head_skb = page_to_skb(vi, rq, page, offset, len, truesize, !xdp_prog);
+    head_skb = page_to_skb(vi, rq, page, offset, len, truesize, !xdp_prog,
+                   metasize);
      curr_skb = head_skb;
        if (unlikely(!curr_skb))

Thank you for your kind review.