Re: [PATCH v2 09/12] virtio: add Rx checksum offload support

[Olivier MATZ <olivier.matz@6wind.com>]

On 10/11/2016 04:36 PM, Maxime Coquelin wrote:
>
>
> On 10/11/2016 04:29 PM, Olivier MATZ wrote:
>>
>>
>> On 10/11/2016 04:04 PM, Maxime Coquelin wrote:
>>>> +/* Optionally fill offload information in structure */
>>>> +static int
>>>> +virtio_rx_offload(struct rte_mbuf *m, struct virtio_net_hdr *hdr)
>>>> +{
>>>> +    struct rte_net_hdr_lens hdr_lens;
>>>> +    uint32_t hdrlen, ptype;
>>>> +    int l4_supported = 0;
>>>> +
>>>> +    /* nothing to do */
>>>> +    if (hdr->flags == 0 && hdr->gso_type == VIRTIO_NET_HDR_GSO_NONE)
>>>> +        return 0;
>>>> +
>>>> +    m->ol_flags |= PKT_RX_IP_CKSUM_UNKNOWN;
>>>> +
>>>> +    ptype = rte_net_get_ptype(m, &hdr_lens, RTE_PTYPE_ALL_MASK);
>>>> +    m->packet_type = ptype;
>>>> +    if ((ptype & RTE_PTYPE_L4_MASK) == RTE_PTYPE_L4_TCP ||
>>>> +        (ptype & RTE_PTYPE_L4_MASK) == RTE_PTYPE_L4_UDP ||
>>>> +        (ptype & RTE_PTYPE_L4_MASK) == RTE_PTYPE_L4_SCTP)
>>>> +        l4_supported = 1;
>>>> +
>>>> +    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 |= PKT_RX_L4_CKSUM_NONE;
>>>> +        } else {
>>>> +            /* Unknown proto or tunnel, do sw cksum. We can assume
>>>> +             * the cksum field is in the first segment since the
>>>> +             * buffers we provided to the host are large enough.
>>>> +             * In case of SCTP, this will be wrong since it's a CRC
>>>> +             * but there's nothing we can do.
>>>> +             */
>>>> +            uint16_t csum, off;
>>>> +
>>>> +            csum = rte_raw_cksum_mbuf(m, hdr->csum_start,
>>>> +                rte_pktmbuf_pkt_len(m) - hdr->csum_start);
>>>> +            if (csum != 0xffff)
>>> Why don't we do the 1-complement if 0xffff?
>>
>> This was modified after a comment from Xiao.
>>
>> In checksum arithmetic (ones' complement), there are 2 equivalent ways
>> to say the checksum is 0: 0xffff (0-), and 0x0000 (0+).
>> Some protocols like UDP use this to differentiate between 0xffff (packet
>> checksum is 0) and 0x0000 (packet checksum is not calculated).
>>
>> Here, we want to avoid to set a checksum to 0, in case it would mean no
>> checksum for UDP packets. Instead, it is set to 0xffff, which is also a
>> valid checksum for this packet.
>
> Ha ok, I wasn't aware of this.
> Thanks for the explanation!
>
> Maybe not a big deal, but we could add likely around the test?

Yep, good idea.

Thanks!
Olivier