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

Xin, Xiaohui
2010-09-15T02:55:51Z

>From: Shirley Ma [mailto:mashirle@us.ibm.com]
>Sent: Wednesday, September 15, 2010 10:41 AM
>To: Xin, Xiaohui
>Cc: Avi Kivity; David Miller; arnd@arndb.de; mst@redhat.com; netdev@vger.kernel.org;
>kvm@vger.kernel.org; linux-kernel@vger.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