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

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Message-ID: <20100915150412.GA679@redhat.com>
Date:	Wed, 15 Sep 2010 17:04:12 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Avi Kivity <avi@...hat.com>,
	"Xin, Xiaohui" <xiaohui.xin@...el.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	kvm@...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, Sep 15, 2010 at 07:52:34AM -0700, Shirley Ma wrote:
> On Wed, 2010-09-15 at 12:10 +0200, Michael S. Tsirkin wrote:
> > Another issue is that macvtap can be bound to almost
> > anything, including e.g. a tap device or a bridge,
> > which might hang on to skb fragments for unlimited time.
> > Zero copy TX won't easily work there.
> > I can imagine either somehow triggering a data copy after the
> > fact (hard), or detecting such devices and avoiding
> > zero copy (unfortunate for guest to guest, and drivers
> > will need tuning).
> 
> So far macvtap zero copy patch is limited to lower devices supports high
> memory DMA, it doesn't apply to a tap device or a bridge.

Okay. What about macvtap in bridge mode?

> > >  Don't you think once I address vhost_add_used_and_signal update
> > > issue, it is a simple and complete patch for macvtap TX zero copy?
> > > 
> > > Thanks
> > > Shirley
> > 
> > I like the fact that the patch is simple. Unfortunately
> > I suspect it'll stop being simple by the time it's complete :) 
> 
> I can make a try. :)
> 
> Thanks
> Shirley
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