On Tue, Sep 14, 2010 at 07:40:52PM -0700, Shirley Ma wrote:

On Wed, 2010-09-15 at 09:50 +0800, Xin, Xiaohui wrote:

quoted

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?

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. 

Thanks
Shirley

For some of the issues, try following the discussion around
net: af_packet: don't call tpacket_destruct_skb() until the skb is sent
out.

Summary: it's difficult to do correctly generally. Limiting ourselves
to transmit on specific devices might make it possible.

-- 
MST