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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 20 Jun 2024 06:19:01 -0400

On Thu, Jun 20, 2024 at 05:28:48PM +0800, Heng Qi wrote:
> On Thu, 20 Jun 2024 16:33:35 +0800, Jason Wang <[email-masked]> wrote:
> > On Tue, Jun 18, 2024 at 11:17 AM Heng Qi <[email-masked]> wrote:
> > >
> > > On Tue, 18 Jun 2024 11:10:26 +0800, Jason Wang <[email-masked]> wrote:
> > > > On Mon, Jun 17, 2024 at 9:15 PM Heng Qi <[email-masked]> 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,
> > 
> > What do you mean by "serve" here? I mean, XDP can calculate the
> > checksum and fill it in the packet by itself.
> > 
> 
> Yes, XDP can parse and calculate checksums for all packets.
> However, the bpf_csum_diff and bpf_l4_csum_replace APIs provided by XDP assume
> that the packets being processed are fully checksumed packets. That is,
> after the XDP program modified the packets, the incremental checksum can be
> calculated (for example, samples/bpf/tcbpf1_kern.c, samples/bpf/test_lwt_bpf.c).
> 
> Therefore, partially checksummed packets cannot be processed normally in these
> examples and need to be discarded.
> 
> > > and this is why we disabled the
> > > offloading of VIRTIO_NET_F_GUEST_CSUM when loading XDP.
> > 
> > If we trust the device to disable VIRTIO_NET_F_GUEST_CSUM, any reason
> > to check VIRTIO_NET_HDR_F_NEEDS_CSUM again in the receive path?
> 
> There doesn't seem to be a mandatory constraint in the spec that devices that
> haven't negotiated VIRTIO_NET_F_GUEST_CSUM cannot set NEEDS_CSUM bit, so I check this.
> 
> Thanks.

The spec says:

\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
  VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can be
  set: if so, the packet checksum at offset \field{csum_offset} 
  from \field{csum_start} and any preceding checksums
  have been validated.  The checksum on the packet is incomplete and
  if bit VIRTIO_NET_HDR_F_RSC_INFO is not set in \field{flags},
  then \field{csum_start} and \field{csum_offset} indicate how to calculate it
  (see Packet Transmission point 1).

So yes, NEEDS_CSUM without VIRTIO_NET_F_GUEST_CSUM is at best undefined.
Please do not try to use it unless VIRTIO_NET_F_GUEST_CSUM is set.

And if you want to be flexible, ignore it unless VIRTIO_NET_F_GUEST_CSUM
has been negotiated.

> > 
> > >
> > > Thanks.
> > 
> > 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 <[email-masked]>
> > > > > ---
> > > > >  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
> > > > >
> > > >
> > >
> >