答复: [dpdk-users] DPDK RX TCP checksum failed

[jiangheng (H)]

> > > On Sat, 26 Mar 2022 08:13:21 +0000
> > > "jiangheng (H)" <jiangheng12 at huawei.com> wrote:
> > >
> > > > Hi all,
> > > >
> > > > I tried using the checksum offloads feature in DPDK and it did not
> > > > see
> > > working under virtual machine.
> > > >
> > > > Port only support TCP checksum and do not support IP checksum:
> > > > rx_offload_capa = DEV_RX_OFFLOAD_TCP_CKSUM tx_offload_capa =
> > > > DEV_TX_OFFLOAD_TCP_CKSUM
> > > >
> > > > so I config rxmode.offload txmode.offloads as below:
> > > > rxmode.offloads = DEV_RX_OFFLOAD_TCP_CKSUM txmode.offloads =
> > > > DEV_TX_OFFLOAD_TCP_CKSUM
> > > >
> > > > For TX, I set the following parameters, it works good.
> > > > mbuf->l2_len = sizeof(*ethhdr)
> > > > mbuf->l3_len = ip header len
> > > > mbuf-ol_flags = RTE_MBUF_F_TX_IPV4 | RTE_MBUF_F_TX_TCP_CKSUM
> > >
> > >
> > > Virtio does not support IP checksum offload.  Because Virtio passes
> > > packets to Linux kernel, and Linux kernel does not do IP checksum offload.
> > > The IP checksum is so trivial it is faster for most things to just
> > > do it in software; the header is only 20 bytes and it will be in cache.
> > >
> > > You should always check device capability before enabling an offload.
> > >
> > >
> > > > For RX, It will execute the following code:
> > > > In drivers/net/virtio/virtio_rxtx.c  virtio_rx_offload function :
> > > >     if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> > > >         hdrlen = hdr_lens.l2_len + hdr_lens.l3_len + hdr_lens.l4_len;
> > > >         if (hdr->csum_start <= hdrlen && l4_supported) {
> > > >             m->ol_flags |= RTE_MBUF_F_RX_L4_CKSUM_NONE;
> > > >         } else {
> > > >
> > > > m->ol_flags set to RTE_MBUF_F_RX_L4_CKSUM_NONE, causing the TCP
> > > RX checksum failed.
> > > > How do I avoid the above code going into this branch?
> > > >
> > >
> > > If you want TCP checksum offload you have to set
> > > RTE_ETH_RX_OFFLOAD_TCP_CKSUM in the rxmode when port is configured.
> > > This will tell virtio to ask the host to do rx offload. Again,
> > > virtio does not do IP checksum offload and you should always query
> > > device capability first.
> >
> > I have queried device capability and it tell me it supports tcp checksum, not
> supports ip checksum.
> > So I have set
> RTE_ETH_RX_OFFLOAD_TCP_CKSUM(DEV_RX_OFFLOAD_TCP_CKSUM)
> > flag when port is configured(use rte_eth_dev_configure function) But RX
> checksum still failed in below branch:
> > drivers/net/virtio/virtio_rxtx.c  virtio_rx_offload function:
> > if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> >         hdrlen = hdr_lens.l2_len + hdr_lens.l3_len + hdr_lens.l4_len;
> >         if (hdr->csum_start <= hdrlen && l4_supported) {
> >                 if (hdr->csum_start <= hdrlen && l4_supported) {
> >                         m->ol_flags |=
> RTE_MBUF_F_RX_L4_CKSUM_NONE;
> >                 } else {
> > Hdr->csum_start <= hdrlen, m->ol_flags will set to
> RTE_MBUF_F_RX_L4_CKSUM_NONE, causing RX checksum failed.
> 
> Could you show the hdr->csum_start and hdrlen value in your test case?
> IIRC, hdr->csum_start should be hdr_lens.l2_len + hdr_lens.l3_len according to
> virtio sepc. The virtio spec has an example as follows:
> 
> ```
> For example, consider a partially checksummed TCP (IPv4) packet. It will have a
> 14 byte ether- net header and 20 byte IP header followed by the TCP header
> (with the TCP checksum field 16 bytes into that header). csum_start will be 14+20
> = 34 (the TCP checksum includes the header), and csum_offset will be 16.
> ```
> 
> --
> Thanks, Harold.

Yes, hdr->csum_start is 34, hdrlen is 54, so m->flags is RTE_MBUF_F_RX_L4_CKSUM_NONE. This cause rx checksum failed. But this line seems to be correct.
I suspect that hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM should not be 1, but I don’t know where this flag is set.