Re: data copy in vhost-user

[Zoltan Kiss <zoltan.kiss-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On 28/04/15 02:22, Xie, Huawei wrote:
>
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces-VfR2kkLFssw@public.gmane.org] On Behalf Of Zoltan Kiss
>> Sent: Tuesday, April 28, 2015 12:27 AM
>> To: Nikita Kalyazin; dev-VfR2kkLFssw@public.gmane.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?
Yes, I think xen-netback had such a version once, but it was removed. As 
far as I know the overhead and complexity of handling these timeouts 
were too severe.
I might be wrong about this, I don't know if this problem applies here 
as well or not.

>>
>> Regards,
>>
>> Zoltan