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

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Message-ID: <1284518452.13351.111.camel@localhost.localdomain>
Date:	Tue, 14 Sep 2010 19:40:52 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Xin, Xiaohui" <xiaohui.xin@...el.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

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

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