On 27/04/15 12:54, Nikita Kalyazin wrote: > Hi, > > > As far as I understand, DPDK vhost-user implementation requires data copy for > either RX or TX (rte_vhost_dequeue_burst() and rte_vhost_enqueue_burst()). It > means that two data copies are needed to transfer a packet from one VM to > another. > > Why is not it possible to eliminate one of the copies (e.g., > rte_vhost_enqueue_burst() might set up a reference at vring descriptor to > mbuf's data rather than copying the data)?
I'm just guessing, but in case of VM-to-VM traffic the receiving one could hold onto the buffer indefinitely, preventing the sender to reuse the buffer. That could lead to a DoS in some cases, and shutting down the sender would be also tricky. At least in case of Xen netback/netfront that's the reason. A reasonable solution for this problem is to make sure the buffer is swapped out with a copy after a finite time. Regards, Zoltan
