On Wed, Jun 01, 2016 at 06:24:18AM +0000, Xie, Huawei wrote: > On 5/3/2016 8:42 AM, Yuanhan Liu wrote: > > Both current kernel virtio driver and DPDK virtio driver use at least > > 2 desc buffer for Tx: the first for storing the header, and the others > > for storing the data. > > Tx could prepend some space for virtio net header whenever possible, so > that it could use only one descriptor.
In such case, it will work as well: it will goto the "else" code path then. > Another thing is this doesn't reduce the check because you also add a check. Actually, yes, it does save check. Before this patch, we have: while (not done yet) { if (desc_is_drain) ...; if (mbuf_is_full) ...; COPY(); } Note that the "while" check will be done twice, therefore, it's 4 checks in total. After this patch, it would be: if (virtio_net_hdr_takes_one_desc) ... while (1) { COPY(); if (desc_is_drain) { break if done; ...; } if (mbuf_is_full { /* small packets will bypass this check */ ....; } } So, for small packets, it takes 2 checks only, which actually saves 2 checks. --yliu