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

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Date:	Wed, 15 Sep 2010 07:27:24 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	"Xin, Xiaohui" <xiaohui.xin@...el.com>,
	Avi Kivity <avi@...hat.com>,
	David Miller <davem@...emloft.net>,
	"arnd@...db.de" <arnd@...db.de>,
	"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

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:
> > 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
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