[dpdk-dev] data copy in vhost-user

Zoltan Kiss zoltan.kiss at linaro.org
Mon Apr 27 18:26:44 CEST 2015



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


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