On 10/13/2016 10:18 AM, Yuanhan Liu wrote:
> On Mon, Oct 03, 2016 at 11:00:23AM +0200, Olivier Matz wrote:
>> +/* When doing TSO, the IP length is not included in the pseudo header
>> + * checksum of the packet given to the PMD, but for virtio it is
>> + * expected.
>> + */
>> +static void
>> +virtio_tso_fix_cksum(struct rte_mbuf *m)
>> +{
>> + /* common case: header is not fragmented */
>> + if (likely(rte_pktmbuf_data_len(m) >= m->l2_len + m->l3_len +
>> + m->l4_len)) {
> ...
>> + /* replace it in the packet */
>> + th->cksum = new_cksum;
>> + } else {
> ...
>> + /* replace it in the packet */
>> + *rte_pktmbuf_mtod_offset(m, uint8_t *,
>> + m->l2_len + m->l3_len + 16) = new_cksum.u8[0];
>> + *rte_pktmbuf_mtod_offset(m, uint8_t *,
>> + m->l2_len + m->l3_len + 17) = new_cksum.u8[1];
>> + }
>
> The tcp header will always be in the mbuf, right? Otherwise, you can't
> update the cksum field here. What's the point of introducing the "else
> clause" then?
Sorry, I don't see the problem you're pointing out here.
What I want to solve here is to support the cases where the mbuf is
segmented in the middle of the network header (which is probably a rare
case).
In the "else" part, I only access the mbuf byte by byte using the
rte_pktmbuf_mtod_offset() accessor. An alternative would have been to
copy the header in a linear buffer, fix the checksum, then copy it again
in the packet, but there is no mbuf helpers to do these copies for now.
Regards,
Olivier
