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
