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



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