netdev - RE: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel

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Message-ID: <F2E9EB7348B8264F86B6AB8151CE2D792B8C81532D@shsmsx502.ccr.corp.intel.com>
Date:	Wed, 15 Sep 2010 10:55:19 +0800
From:	"Xin, Xiaohui" <xiaohui.xin@...el.com>
To:	Shirley Ma <mashirle@...ibm.com>
CC:	Avi Kivity <avi@...hat.com>, David Miller <davem@...emloft.net>,
	"arnd@...db.de" <arnd@...db.de>, "mst@...hat.com" <mst@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host
 kernel

>From: Shirley Ma [mailto:mashirle@...ibm.com]
>Sent: Wednesday, September 15, 2010 10:41 AM
>To: Xin, Xiaohui
>Cc: Avi Kivity; David Miller; arnd@...db.de; mst@...hat.com; netdev@...r.kernel.org;
>kvm@...r.kernel.org; linux-kernel@...r.kernel.org
>Subject: RE: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel
>
>On Wed, 2010-09-15 at 09:50 +0800, Xin, Xiaohui wrote:
>> I think what David said is what we have thought before in mp device.
>> Since we are not sure the exact time the tx buffer was wrote though
>> DMA operation.
>> But the deadline is when the tx buffer was freed. So we only notify
>> the vhost stuff
>> about the write when tx buffer freed. But the deadline is maybe too
>> late for performance.
>
>Have you tried it? If so what's the performance penalty you have seen by
>notifying vhost when tx buffer freed?
>

We did not try it before, as we cared RX side more.

>I am thinking to have a callback in skb destructor,
>vhost_add_used_and_signal gets updated when skb is actually freed, vhost
>vq & head need to be passed to the callback. This might requires vhost
>ring size is at least as big as the lower device driver.
>

That's almost the same what we have done except we use destructor_arg and
another callback..

>Thanks
>Shirley

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