[dpdk-dev] [PATCH v2 09/12] virtio: add Rx checksum offload support

Olivier MATZ olivier.matz at 6wind.com
Tue Oct 11 16:29:50 CEST 2016



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.

Regards,
Olivier


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