netdev - Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice

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Date:	Thu, 19 May 2011 12:42:49 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Michał Mirosław <mirqus@...il.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in
 netdevice

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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