Re: [PATCH 2/2] virtio_net: fixing XDP for fully checksummed packets handling

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

On Tue, 18 Jun 2024 11:10:26 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Mon, Jun 17, 2024 at 9:15 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
> >
> > The XDP program can't correctly handle partially checksummed
> > packets, but works fine with fully checksummed packets.
> 
> Not sure this is ture, if I was not wrong, XDP can try to calculate checksum.

XDP's interface serves a full checksum, and this is why we disabled the
offloading of VIRTIO_NET_F_GUEST_CSUM when loading XDP.

Thanks.

> 
> Thanks
> 
> > If the
> > device has already validated fully checksummed packets, then
> > the driver doesn't need to re-validate them, saving CPU resources.
> >
> > Additionally, the driver does not drop all partially checksummed
> > packets when VIRTIO_NET_F_GUEST_CSUM is not negotiated. This is
> > not a bug, as the driver has always done this.
> >
> > Fixes: 436c9453a1ac ("virtio-net: keep vnet header zeroed after processing XDP")
> > Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> > ---
> >  drivers/net/virtio_net.c | 20 +++++++++++++++++++-
> >  1 file changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index aa70a7ed8072..ea10db9a09fa 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1360,6 +1360,10 @@ static struct sk_buff *receive_small_xdp(struct net_device *dev,
> >         if (unlikely(hdr->hdr.gso_type))
> >                 goto err_xdp;
> >
> > +       /* Partially checksummed packets must be dropped. */
> > +       if (unlikely(hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
> > +               goto err_xdp;
> > +
> >         buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
> >                 SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> >
> > @@ -1677,6 +1681,10 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
> >         if (unlikely(hdr->hdr.gso_type))
> >                 return NULL;
> >
> > +       /* Partially checksummed packets must be dropped. */
> > +       if (unlikely(hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
> > +               return NULL;
> > +
> >         /* 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
> > @@ -1943,6 +1951,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> >         struct net_device *dev = vi->dev;
> >         struct sk_buff *skb;
> >         struct virtio_net_common_hdr *hdr;
> > +       u8 flags;
> >
> >         if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
> >                 pr_debug("%s: short packet %i\n", dev->name, len);
> > @@ -1951,6 +1960,15 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> >                 return;
> >         }
> >
> > +       /* 1. Save the flags early, as the XDP program might overwrite them.
> > +        * These flags ensure packets marked as VIRTIO_NET_HDR_F_DATA_VALID
> > +        * stay valid after XDP processing.
> > +        * 2. XDP doesn't work with partially checksummed packets (refer to
> > +        * virtnet_xdp_set()), so packets marked as
> > +        * VIRTIO_NET_HDR_F_NEEDS_CSUM get dropped during XDP processing.
> > +        */
> > +       flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
> > +
> >         if (vi->mergeable_rx_bufs)
> >                 skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
> >                                         stats);
> > @@ -1966,7 +1984,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> >         if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
> >                 virtio_skb_set_hash(&hdr->hash_v1_hdr, skb);
> >
> > -       if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
> > +       if (flags & VIRTIO_NET_HDR_F_DATA_VALID)
> >                 skb->ip_summed = CHECKSUM_UNNECESSARY;
> >
> >         if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
> > --
> > 2.32.0.3.g01195cf9f
> >
>