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

[Harold Huang <baymaxhuang@gmail.com>]

On Mon, Mar 28, 2022 at 9:50 AM jiangheng (H) <jiangheng12@huawei.com> wrote:
>
> > On Sat, 26 Mar 2022 08:13:21 +0000
> > "jiangheng (H)" <jiangheng12@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.