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

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Date:	Wed, 15 Sep 2010 12:10:00 +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 Tue, Sep 14, 2010 at 11:21:15PM -0700, Shirley Ma wrote:
> On Wed, 2010-09-15 at 07:12 +0200, Michael S. Tsirkin wrote:
> > Yes, I agree this patch is useful for demo purposes:
> > simple, and shows what kind of performance gains
> > we can expect for TX. 
> 
> Any other issue you can see in this patch beside vhost descriptors
> update?

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).

>  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 :)

-- 
MST
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