Date: Thu, 29 Jun 2023 12:11:57 +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 Thu, Jun 29, 2023 at 12:03 PM Jason Wang <jasowang@...hat.com> wrote:
>
> On Wed, Jun 28, 2023 at 6:02 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
> >
> >
> >
> > 在 2023/6/28 下午2:50, Jason Wang 写道:
> > > 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.
> >
> > Yes. It works fine after skb_checksum_setup().
> >
> > >
> > >>>>> 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.
> >
> > Yes.
> >
> > >
> > >>> 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
> > >
> > > ?
> >
> > I don't think we need to make many assumptions about the behavior of XDP
> > programs.
> > Because we are out of control for various users using XDP.
>
> Exactly, but this patch seems to assume the XDP behaviour as you said previously
>
> """
> > >>>> 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,
> """
>
> ?
>
> >
> > The core purpose of this patch is to:
> > #1 Solve the packet loss problem caused by loading XDP between vm-vm on
> > the same host (scenario with partial checksum).
>
> So we disabled guest_csum and the host (e.g TAP) will do checksum for
> us. Otherwise it should be a bug of the host.
>
> Thanks

Btw, it looks to me that this patch doesn't fix the XDP_TX path?
Should we do that or it's not related at all?

Thanks

>
> > #2 For scenarios other than #1, virtio-net with this patch is already
> > consistent with other existing NIC drivers (simple such as
> > ixgbe[1]/bnxt[2]/mvneta[3]/..):
> > the rx side only needs to have NETIF_F_RXCSUM and the device has
> > verified the packet has a valid checksum.
> > Then skb converted from xdp_buff (XDP returns XDP_PASS) can have
> > skb->ip_summed = CHECKSUM_UNNECESSARY.
> >
> > If the comment for DATA_VALID is confusing, I'll just remove it.
> >
> > [1] ixgbe_clean_rx_irq()-> ixgbe_run_xdp()-> ixgbe_process_skb_fields()
> > ->ixgbe_rx_checksum()
> > [2] bnxt_xdp_build_skb()
> > [3] mvneta_swbm_build_skb
> >
> > Thanks.
> >
> > >
> > > 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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