Re: [PATCH net-next v4 1/2] virtio-net: support coexistence of XDP and GUEST_CSUM

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On Wed, Jun 28, 2023 at 11:05 AM Heng Qi <hengqi@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> ---
> 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:

1) probe the csum_start/offset
2) leave it as CHECKSUM_PARTIAL

?

> +       } 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.

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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