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Message-ID: <CACGkMEu=Cs5DFP+EFqxUXaiqz7vewhQ5zMMtChGpR_oGjrvMCg@mail.gmail.com>
Date: Wed, 28 Jun 2023 12:02:17 +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 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?
>
> >
> > 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? Btw, is there a way for
the XDP program to know whether the csum has been verified by the
device? ( I guess not).
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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