Re: [dpdk-dev] data copy in vhost-user

["Xie, Huawei" <huawei.xie@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Zoltan Kiss
> Sent: Tuesday, April 28, 2015 12:27 AM
> To: Nikita Kalyazin; dev@dpdk.org
> Subject: Re: [dpdk-dev] data copy in vhost-user
> 
> 
> 
> 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)?
This had been added to the to-do list. We could delay the copy until the real copy is needed.
> 
> 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.
Do you mean we associate a timeout for the buffer?
> 
> Regards,
> 
> Zoltan