Date: Mon, 3 Jul 2023 14:33:55 +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 Mon, Jul 3, 2023 at 12:48 AM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>
>
>
> 在 2023/6/30 下午5:57, Heng Qi 写道:
> > On Fri, Jun 30, 2023 at 03:35:08PM +0800, Jason Wang wrote:
> >> On Thu, Jun 29, 2023 at 1:47 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
> >>> On Thu, Jun 29, 2023 at 12:03:28PM +0800, Jason Wang 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
> >>>>
> >>> Let me sum it up:
> >>> When getting a skb from XDP_PASS, rx virtio-net will have the following
> >>> scenarios:
> >>> 1. In a virtualized environment such as vm-vhost_user-vm, vhost-user or
> >>> some backends find that no physical link is required to reach the
> >>> destination, then they will save the cost of calculating the complete
> >>> checksum on the tx device side. That is, the device
> >>> directly sends the packet with the pseudo-header checksum, and the
> >>> rx side directly receives the packet with the pseudo-header checksum.
> >>> This is a problem we have to deal with, but other hardware
> >>> NICs don't have this problem (veth actually also has this problem,
> >>> which I mentioned in the proposal).
> >>>
> >>> 2. If the packet has passed through the physical link, it means
> >>> that the received packet has a complete checksum and not only a
> >>> pseudo-header checksum. At this time, virtio-net only needs to be
> >>> consistent with other NIC driver behaviors:
> >>>
> >>>    2.1 If the device has verified the checksum, mark
> >>>        skb->ip_summend=CHECKSUM_UNNECESSARY after XDP processing;
> >>>    2.2 If the device does not verify the checksum, after XDP processing,
> >>>        skb->ip_summend=CHECKSUM_NONE, the checksum is verified by the stack.
> >> This doesn't answer my question, let me ask you differently.
> >>
> >> Let's take xdp_tx_iptunnel_kern.c as an example. With your patch, can
> >> it work when we receive a packet with partial csum? If not, it breaks
> > It works fine.
>
> Oh! Sorry for the noise, I misunderstood your scenario. The answer is
> that XDP_TX will cause packet loss in this scenario.
>
> For XDP_TX, flags in virtnet hdr are reset to 0, so if there is XDP on
> the rx side, the process of converting xdp_buff (if XDP_PASS)
> to skb will reset skb->csum_{start, offset} to 0. Therefore, the packets
> carrying partial csum sent out by XDP_TX will be lost
> (with or without this patch, the result is the same).

Without this patch, we won't receive a partial csum packet since
guest_csum is disabled.

> This situation is actually similar to XDP_PASS without this patch: all
> XDP_PASS packets carrying partial csum will also be dropped.
>
> Therefore, I think the XDP program is mainly designed for packets with
> complete checksums (XDP does not have a design for packets
> with partial csum, which brings about the problem that this patch is
> trying to solve). This test(xdp_tx_iptunnel_kern.c) causes no packet
> loss when the packet has a complete checksum.
>
> The XDP_TX situation is complicated when the checksum of the packet is
> partial csum.
> So the problem temporarily solved by this patch is: the packet loss
> problem when the checksum of the data packet is partial csum
> (flags in virtnethdr is marked as NEEDS_CSUM ) and XDP_PASS is converted
> to skb.

I'm not sure I get this, do you mean we fallback to xdp generic? Looks
like one way to go.

Thanks

>
> Thanks!
>
> >
> > When XDP (mean xdp_tx_iptunnel_kern.c) encapsulates the qualified
> > (by querying vip2tnl maps) packets into IPIP tunnel packets. And
> > it calculates the checksum for its new outer IP header (IP checksum
> > verification is always done by the stack, without unloading, see
> > ip_rcv_core()->ip_fast_csum())
> >       for (i = 0; i < sizeof(*iph) >> 1; i++)
> >            csum += *next_iph_u16++;
> >        iph->check = ~((csum & 0xffff) + (csum >> 16));
> >
> > Finally, in the packet sent out by XDP_TX:
> > 1. flags in virtio_net_hdr is 0.
> > 2. The inner IP checksum (old_ip->check) in the original payload remains unchanged.
> > 3. The checksum of the inner transport (udp/tcp->check) remains unchanged.
> > 4. skb->ip_summed remains unchanged. Still CHECKSUM_PARTIAL.
> >
> > After the receiving side (with this patch) receives this IP tunnel packet,
> > it finds that the flags of virtio_net_hdr is 0, and submits it to the
> > stack. Then the stack verifies that:
> > 1. the outer ip checksum is correct (the XDP program calculated the checksum of the outer IP before).
> > 2. The inner IP is correct.
> > 3. udp/tcp is verified by skb_csum_unnecessary() and passed to continue
> > to pass.
> >
> > In fact, this patch does not affect the behavior of XDP_TX.
> >
> > Thanks!
> >
> >> existing applications.
> >>
> >> Thanks
> >>
> >>> Thanks.
> >>>
> >>>
> >>>> """
> >>>>>>>>> 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
> >>>>
> >>>>> #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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