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

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Message-ID: <20100929081645.GA21195@redhat.com>
Date:	Wed, 29 Sep 2010 10:16:45 +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 28, 2010 at 08:24:29PM -0700, Shirley Ma wrote:
> Hello Michael,
> 
> On Wed, 2010-09-15 at 07:52 -0700, Shirley Ma wrote:
> > > >  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. :)
> 
> I compared several approaches for addressing the issue being raised here
> on how/when to update vhost_add_used_and_signal. The simple approach I
> have found is:
> 
> 1. Adding completion field in struct virtqueue;
> 2. when it is a zero copy packet, put vhost thread wait for completion
> to update vhost_add_used_and_signal;
> 3. passing vq from vhost to macvtap as skb destruct_arg;
> 4. when skb is freed for the last reference, signal vq completion
> The test results show same performance as the original patch. How do you
> think? If it sounds good to you. I will resubmit this reversion patch.
> The patch still keeps as simple as it was before. :)
> 
> Thanks
> Shirley

If you look at dev_hard_start_xmit you will see a call
to skb_orphan_try which often calls the skb destructor.
So I suspect this is almost equivalent to your original patch,
and has the same correctness issue.

-- 
MST
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