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? --yliu