Date:   Wed, 28 Jun 2023 14:50:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Heng Qi <hengqi@...ux.alibaba.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        "Michael S . Tsirkin" <mst@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH net-next v4 1/2] virtio-net: support coexistence of XDP
 and GUEST_CSUM

On Wed, Jun 28, 2023 at 12:56 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>
> On Wed, Jun 28, 2023 at 12:02:17PM +0800, Jason Wang wrote:
> > On Wed, Jun 28, 2023 at 11:42 AM Heng Qi <hengqi@...ux.alibaba.com> wrote:
> > >
> > >
> > >
> > > 在 2023/6/28 上午11:22, Jason Wang 写道:
> > > > On Wed, Jun 28, 2023 at 11:05 AM Heng Qi <hengqi@...ux.alibaba.com> wrote:
> > > >> We are now re-probing the csum related fields and trying
> > > >> to have XDP and RX hw checksum capabilities coexist on the
> > > >> XDP path. For the benefit of:
> > > >> 1. RX hw checksum capability can be used if XDP is loaded.
> > > >> 2. Avoid packet loss when loading XDP in the vm-vm scenario.
> > > >>
> > > >> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
> > > >> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > > >> ---
> > > >> v3->v4:
> > > >>    - Rewrite some comments.
> > > >>
> > > >> v2->v3:
> > > >>    - Use skb_checksum_setup() instead of virtnet_flow_dissect_udp_tcp().
> > > >>      Essentially equivalent.
> > > >>
> > > >>   drivers/net/virtio_net.c | 82 +++++++++++++++++++++++++++++++++-------
> > > >>   1 file changed, 69 insertions(+), 13 deletions(-)
> > > >>
> > > >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > >> index 5a7f7a76b920..a47342f972b5 100644
> > > >> --- a/drivers/net/virtio_net.c
> > > >> +++ b/drivers/net/virtio_net.c
> > > >> @@ -1568,6 +1568,41 @@ static void virtio_skb_set_hash(const struct virtio_net_hdr_v1_hash *hdr_hash,
> > > >>          skb_set_hash(skb, __le32_to_cpu(hdr_hash->hash_value), rss_hash_type);
> > > >>   }
> > > >>
> > > >> +static int virtnet_set_csum_after_xdp(struct virtnet_info *vi,
> > > >> +                                     struct sk_buff *skb,
> > > >> +                                     __u8 flags)
> > > >> +{
> > > >> +       int err = 0;
> > > >> +
> > > >> +       /* When XDP program is loaded, the vm-vm scenario on the same host,
> > > >> +        * packets marked VIRTIO_NET_HDR_F_NEEDS_CSUM without a complete checksum
> > > >> +        * will travel. Although these packets are safe from the point of
> > > >> +        * view of the vm, in order to be successfully forwarded on the upper
> > > >> +        * layer and to avoid packet loss caused by XDP modification,
> > > >> +        * we re-probe the necessary checksum related information:
> > > >> +        * skb->csum_{start, offset}, pseudo-header checksum.
> > > >> +        *
> > > >> +        * If the received packet is marked VIRTIO_NET_HDR_F_DATA_VALID:
> > > >> +        * when _F_GUEST_CSUM is negotiated, the device validates the checksum
> > > >> +        * and virtio-net sets skb->ip_summed to CHECKSUM_UNNECESSARY;
> > > >> +        * otherwise, virtio-net hands over to the stack to validate the checksum.
> > > >> +        */
> > > >> +       if (flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> > > >> +               /* No need to care about SCTP because virtio-net currently doesn't
> > > >> +                * support SCTP CRC checksum offloading, that is, SCTP packets have
> > > >> +                * complete checksums.
> > > >> +                */
> > > >> +               err = skb_checksum_setup(skb, true);
> > > > A second thought, any reason why a checksum is a must here. Could we simply:
> > >
> > > When net.ipv4.ip_forward sysctl is enabled, such packets may be
> > > forwarded (return to the tx path) at the IP layer.
> > > If the device has the tx hw checksum offloading cap, packets will have
> > > complete checksums based on our calculated 'check' value.
> >
> > Actually, I mean why can't we offload the checksum to the hardware in this case?
>
> Yes that's what I explained:)
>
> Checksum of udp/tcp includes the pseudo-header checksum and the checksum of the entire udp/tcp payload.
> When tx checksum offloading is enabled, the upper layer will only calculate the pseudo-header checksum,
> and the rest of the checksum of the entire udp/tcp payload will be calculated by hardware.
>
>
> Please see udp_send_skb():
>
> "
>         } else if (skb->ip_summed == CHECKSUM_PARTIAL) { /* UDP hardware csum */
> csum_partial:
>
>                 udp4_hwcsum(skb, fl4->saddr, fl4->daddr);
>                 goto send;
>
>         } else
>                 csum = udp_csum(skb);
>
>         /* add protocol-dependent pseudo-header */
>         uh->check = csum_tcpudp_magic(fl4->saddr, fl4->daddr, len,
>                                       sk->sk_protocol, csum);
>         if (uh->check == 0)
>                 uh->check = CSUM_MANGLED_0;
>
> send:
>         err = ip_send_skb(sock_net(sk), skb);
> "

Ok, so I think what I missed is that the CHECKSUM_PARTIAL is set up by
skb_checksum_setup() so we don't even need to care about that.

>
> >
> > >
> > > >
> > > > 1) probe the csum_start/offset
> > > > 2) leave it as CHECKSUM_PARTIAL
> > > >
> > > > ?
> > >
> > > The reason is as I explained above.
> > >
> > > >
> > > >> +       } else if (flags & VIRTIO_NET_HDR_F_DATA_VALID) {
> > > >> +               /* XDP guarantees that packets marked as VIRTIO_NET_HDR_F_DATA_VALID
> > > >> +                * still have correct checksum after they are processed.
> > > >> +                */
> > > > Do you mean it's the charge of the XDP program to calculate the csum
> > > > in this case? Seems strange.
> > >
> > > Packet with complete checksum (and has been verified by rx device
> > > because it has VIRTIO_NET_HDR_F_DATA_VALID)
> > > when modified by XDP, XDP program should use the helper provided by XDP
> > > core to make the checksum correct,
> >
> > Could you give me a pointer to that helper?
>
> bpf_csum_diff(),

Ok.

> bpf_{l3,l4}_csum_replace()

This seems not to be a helpr for XDP but for other bpf like cls.

>
> >Btw, is there a way for
> > the XDP program to know whether the csum has been verified by the
> > device? ( I guess not).
> >
>
> Not. But we only do this (mark skb->ip_summed = CHECKSUM_UNNECESSARY) for packets with VIRTIO_NET_HDR_F_DATA_VALID now.

So if I understand you correctly, you meant for the XDP program that
wants to modify the packet:

1) check whether the checksum is valid
2) if yes, recalculate the checksum after the modification
3) if not, just do nothing for the checksum and the driver need to
re-probe the csum_start/offset

?

Thanks

>
> Thanks.
>
> > Thanks
> >
> >
> > > otherwise, VIRTIO_NET_HDR_F_DATA_VALID has been cleared and skb
> > > ->ip_summed=CHECKSUM_NONE, so the stack
> > > will re-verify the checksum, causing packet loss due to wrong checksum.
> > >
> > > Thanks.
> > >
> > > >
> > > > Thanks
> > > >
> > > >> +               skb->ip_summed = CHECKSUM_UNNECESSARY;
> > > >> +       }
> > > >> +
> > > >> +       return err;
> > > >> +}
> > > >> +
> > > >>   static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > > >>                          void *buf, unsigned int len, void **ctx,
> > > >>                          unsigned int *xdp_xmit,
> > > >> @@ -1576,6 +1611,7 @@ 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_hdr_mrg_rxbuf *hdr;
> > > >> +       __u8 flags;
> > > >>
> > > >>          if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
> > > >>                  pr_debug("%s: short packet %i\n", dev->name, len);
> > > >> @@ -1584,6 +1620,12 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > > >>                  return;
> > > >>          }
> > > >>
> > > >> +       /* XDP may modify/overwrite the packet, including the virtnet hdr,
> > > >> +        * so save the flags of the virtnet hdr before XDP processing.
> > > >> +        */
> > > >> +       if (unlikely(vi->xdp_enabled))
> > > >> +               flags = ((struct virtio_net_hdr_mrg_rxbuf *)buf)->hdr.flags;
> > > >> +
> > > >>          if (vi->mergeable_rx_bufs)
> > > >>                  skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
> > > >>                                          stats);
> > > >> @@ -1595,23 +1637,37 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > > >>          if (unlikely(!skb))
> > > >>                  return;
> > > >>
> > > >> -       hdr = skb_vnet_hdr(skb);
> > > >> -       if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
> > > >> -               virtio_skb_set_hash((const struct virtio_net_hdr_v1_hash *)hdr, skb);
> > > >> -
> > > >> -       if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
> > > >> -               skb->ip_summed = CHECKSUM_UNNECESSARY;
> > > >> +       if (unlikely(vi->xdp_enabled)) {
> > > >> +               /* Required to do this before re-probing and calculating
> > > >> +                * the pseudo-header checksum.
> > > >> +                */
> > > >> +               skb->protocol = eth_type_trans(skb, dev);
> > > >> +               skb_reset_network_header(skb);
> > > >> +               if (virtnet_set_csum_after_xdp(vi, skb, flags) < 0) {
> > > >> +                       pr_debug("%s: errors occurred in setting partial csum",
> > > >> +                                dev->name);
> > > >> +                       goto frame_err;
> > > >> +               }
> > > >> +       } else {
> > > >> +               hdr = skb_vnet_hdr(skb);
> > > >> +               if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
> > > >> +                       virtio_skb_set_hash((const struct virtio_net_hdr_v1_hash *)hdr, skb);
> > > >> +
> > > >> +               if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
> > > >> +                       skb->ip_summed = CHECKSUM_UNNECESSARY;
> > > >> +
> > > >> +               if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
> > > >> +                                         virtio_is_little_endian(vi->vdev))) {
> > > >> +                       net_warn_ratelimited("%s: bad gso: type: %u, size: %u\n",
> > > >> +                                            dev->name, hdr->hdr.gso_type,
> > > >> +                                            hdr->hdr.gso_size);
> > > >> +                       goto frame_err;
> > > >> +               }
> > > >>
> > > >> -       if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
> > > >> -                                 virtio_is_little_endian(vi->vdev))) {
> > > >> -               net_warn_ratelimited("%s: bad gso: type: %u, size: %u\n",
> > > >> -                                    dev->name, hdr->hdr.gso_type,
> > > >> -                                    hdr->hdr.gso_size);
> > > >> -               goto frame_err;
> > > >> +               skb->protocol = eth_type_trans(skb, dev);
> > > >>          }
> > > >>
> > > >>          skb_record_rx_queue(skb, vq2rxq(rq->vq));
> > > >> -       skb->protocol = eth_type_trans(skb, dev);
> > > >>          pr_debug("Receiving skb proto 0x%04x len %i type %i\n",
> > > >>                   ntohs(skb->protocol), skb->len, skb->pkt_type);
> > > >>
> > > >> --
> > > >> 2.19.1.6.gb485710b
> > > >>
> > >
>

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