[dpdk-dev] [PATCH v2 12/12] virtio: add Tso support

[Olivier MATZ]

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