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From: Zoltan Kiss <zoltan.kiss-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
To: Nikita Kalyazin
	<n.kalyazin-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>,
	dev-VfR2kkLFssw@public.gmane.org
Subject: Re: data copy in vhost-user
Date: Mon, 27 Apr 2015 17:26:44 +0100	[thread overview]
Message-ID: <553E6344.6030104@linaro.org> (raw)
In-Reply-To: <20150427115422.GI28809-dOcb+p3lB4ioPJ7u4KOs7jOMi+9Jb/nM@public.gmane.org>



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

  parent reply	other threads:[~2015-04-27 16:26 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-04-27 11:54 data copy in vhost-user Nikita Kalyazin
     [not found] ` <20150427115422.GI28809-dOcb+p3lB4ioPJ7u4KOs7jOMi+9Jb/nM@public.gmane.org>
2015-04-27 16:26   ` Zoltan Kiss [this message]
     [not found]     ` <553E6344.6030104-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
2015-04-28  1:22       ` Xie, Huawei
     [not found]         ` <C37D651A908B024F974696C65296B57B0F45DBE0-0J0gbvR4kThpB2pF5aRoyrfspsVTdybXVpNB7YpNyf8@public.gmane.org>
2015-04-28 12:24           ` Zoltan Kiss
     [not found]             ` <553F7BF9.1010505-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
2015-04-29  7:41               ` Nikita Kalyazin

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