On Thu, Oct 13, 2016 at 05:15:24PM +0200, Olivier MATZ wrote: > > > On 10/13/2016 05:01 PM, Yuanhan Liu wrote: > >On Thu, Oct 13, 2016 at 04:52:25PM +0200, Olivier MATZ wrote: > >> > >> > >>On 10/13/2016 04:16 PM, Yuanhan Liu wrote: > >>>On Thu, Oct 13, 2016 at 04:02:49PM +0200, Olivier MATZ wrote: > >>>> > >>>> > >>>>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). > >>> > >>>How it's gonna segmented? > >> > >>The mbuf is given by the application. So if the application generates a > >>segmented mbuf, it should work. > >> > >>This could happen for instance if the application uses mbuf clones to share > >>the IP/TCP/data part of the mbuf and prepend a specific Ethernet/vlan for > >>different destination. > >> > >> > >>>>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. > >>> > >>>In the "else" clause, the ip header is still in the mbuf, right? > >>>Why do you have to access it the way like: > >>> > >>> ip_version = *rte_pktmbuf_mtod_offset(m, uint8_t *, > >>> m->l2_len) >> 4; > >>> > >>>Why can't you just use > >>> > >>> iph = rte_pktmbuf_mtod_offset(m, struct ipv4_hdr *, m->l2_len); > >>> iph->version_ihl ....; > >> > >>AFAIK, there is no requirement that each network header has to be contiguous > >>in a mbuf segment. > >> > >>Of course, a split in the middle of a network header probably never > >>happens... but we never knows, as it is not forbidden. I think the code > >>should be robust enough to avoid accesses to wrong addresses. > >> > >>Hope it's clear enough :) > > > >Thanks, but not really. Maybe let me ask this way: what wrong would > >happen if we use > > iph = rte_pktmbuf_mtod_offset(m, struct ipv4_hdr *, m->l2_len); > >to access the IP header? Is it about the endian? > > If you have a packet split like this: > > mbuf segment 1 mbuf segment 2 > ---------------------------- ------------------------------ > | Ethernet header | IP hea| |der | TCP header | data > ---------------------------- ------------------------------ > ^ > iph
Thanks, that's clear. How could you be able to access the tcp header from the first mbuf then? I mean, how is the following code supposed to work? prev_cksum.u8[0] = *rte_pktmbuf_mtod_offset(m, uint8_t *, m->l2_len + m->l3_len + 16); > The IP header is not contiguous. So accessing to the end of the structure > will access to a wrong location. > > >One more question is do you have any case to trigger the "else" clause? > > No, but I think it may happen. A piece of untest code is not trusted though ... --yliu