virtio_net: Add XDP frame size in two code paths

The virtio_net driver is running inside the guest-OS. There are two
XDP receive code-paths in virtio_net, namely receive_small() and
receive_mergeable(). The receive_big() function does not support XDP.

In receive_small() the frame size is available in buflen. The buffer
backing these frames are allocated in add_recvbuf_small() with same
size, except for the headroom, but tailroom have reserved room for
skb_shared_info. The headroom is encoded in ctx pointer as a value.

In receive_mergeable() the frame size is more dynamic. There are two
basic cases: (1) buffer size is based on a exponentially weighted
moving average (see DECLARE_EWMA) of packet length. Or (2) in case
virtnet_get_headroom() have any headroom then buffer size is
PAGE_SIZE. The ctx pointer is this time used for encoding two values;
the buffer len "truesize" and headroom. In case (1) if the rx buffer
size is underestimated, the packet will have been split over more
buffers (num_buf info in virtio_net_hdr_mrg_rxbuf placed in top of
buffer area). If that happens the XDP path does a xdp_linearize_page
operation.

V3: Adjust frame_sz in receive_mergeable() case, spotted by Jason Wang.

The code is really hard to follow, so some hints to reviewers.
The receive_mergeable() case gets frames that were allocated in
add_recvbuf_mergeable() which uses headroom=virtnet_get_headroom(),
and 'buf' ptr is advanced this headroom.  The headroom can only
be 0 or VIRTIO_XDP_HEADROOM, as virtnet_get_headroom is really
simple:

  static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
  {
	return vi->xdp_queue_pairs ? VIRTIO_XDP_HEADROOM : 0;
  }

As frame_sz is an offset size from xdp.data_hard_start, reviewers
should notice how this is calculated in receive_mergeable():

  int offset = buf - page_address(page);
  [...]
  data = page_address(xdp_page) + offset;
  xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;

The calculated offset will always be VIRTIO_XDP_HEADROOM when
reaching this code.  Thus, xdp.data_hard_start will be page-start
address plus vi->hdr_len.  Given this xdp.frame_sz need to be
reduced with vi->hdr_len size.

IMHO a followup patch should cleanup this code to make it easier
to maintain and understand, but it is outside the scope of this
patchset.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/bpf/158945344436.97035.9445115070189151680.stgit@firesoul

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 11f7224605133aa1ee1eede78a63c2dfc02fc201..9e1b5d748586f7943b5a42f6ce1a99f53098822c 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -689,6 +689,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
                xdp.data_end = xdp.data + len;
                xdp.data_meta = xdp.data;
                xdp.rxq = &rq->xdp_rxq;
+               xdp.frame_sz = buflen;
                orig_data = xdp.data;
                act = bpf_prog_run_xdp(xdp_prog, &xdp);
                stats->xdp_packets++;
@@ -797,10 +798,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
        int offset = buf - page_address(page);
        struct sk_buff *head_skb, *curr_skb;
        struct bpf_prog *xdp_prog;
-       unsigned int truesize;
+       unsigned int truesize = mergeable_ctx_to_truesize(ctx);
        unsigned int headroom = mergeable_ctx_to_headroom(ctx);
-       int err;
        unsigned int metasize = 0;
+       unsigned int frame_sz;
+       int err;
 
        head_skb = NULL;
        stats->bytes += len - vi->hdr_len;
@@ -821,6 +823,11 @@ 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()
+                */
+               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
@@ -834,6 +841,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
                                                      page, offset,
                                                      VIRTIO_XDP_HEADROOM,
                                                      &len);
+                       frame_sz = PAGE_SIZE;
+
                        if (!xdp_page)
                                goto err_xdp;
                        offset = VIRTIO_XDP_HEADROOM;
@@ -850,6 +859,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
                xdp.data_end = xdp.data + (len - vi->hdr_len);
                xdp.data_meta = xdp.data;
                xdp.rxq = &rq->xdp_rxq;
+               xdp.frame_sz = frame_sz - vi->hdr_len;
 
                act = bpf_prog_run_xdp(xdp_prog, &xdp);
                stats->xdp_packets++;
@@ -924,7 +934,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
        }
        rcu_read_unlock();
 
-       truesize = mergeable_ctx_to_truesize(ctx);
        if (unlikely(len > truesize)) {
                pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
                         dev->name, len, (unsigned long)ctx);