Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in
 netdevice
From: Shirley Ma <mashirle@us.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: =?UTF-8?Q?Micha=C5=82_Miros=C5=82aw?= <mirqus@gmail.com>,
        Ben Hutchings <bhutchings@solarflare.com>,
        David Miller <davem@davemloft.net>,
        Eric Dumazet <eric.dumazet@gmail.com>, Avi Kivity <avi@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, netdev@vger.kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
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Date: Thu, 19 May 2011 12:42:49 -0700
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On Wed, 2011-05-18 at 10:00 -0700, Shirley Ma wrote:
> On Wed, 2011-05-18 at 19:51 +0300, Michael S. Tsirkin wrote:
> > > > Yes, I agree.  I think for tcpdump, we really need to copy the
> > data
> > > > anyway, to avoid guest changing it in between.  So we do that
> and
> > then
> > > > use the copy everywhere, release the old one. Hmm? 
> > > 
> > > Yes. Old one use zerocopy, new one use copy data.
> > > 
> > > Thanks
> > > Shirley
> > 
> > No, that's wrong, as they might become different with a
> > malicious guest. As long as we copied already, lets realease
> > the data and have everyone use the copy. 
> 
> Ok, I will patch pskb_expand_head to test it out. 

I am patching skb_copy, skb_clone, pskb_copy, pskb_expand_head to
convert a zero-copy skb to a copy skb to avoid this kind of issue.

This overhead won't impact macvtap/vhost TX zero-copy normally.

Shirley

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