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

[Stephen Hemminger <stephen@networkplumber.org>]

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.