Subject: Re: [PATCH V3 2/8] Add a new zerocopy device flag
From: Shirley Ma <mashirle@us.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: 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
In-Reply-To: <20110502195302.GA25619@redhat.com>
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Date: Tue, 03 May 2011 10:42:50 -0700
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On Mon, 2011-05-02 at 22:53 +0300, Michael S. Tsirkin wrote:
> On Mon, May 02, 2011 at 11:47:08AM -0700, Shirley Ma wrote:
> > On Mon, 2011-05-02 at 13:42 +0300, Michael S. Tsirkin wrote:
> > > This comment should specify what exactly is the promise the
> > > device makes by setting this flag. Specifically, the
> > > condition is that no skb fragments are used
> > > after the uinfo callback has been called.
> > > 
> > > The way it's implemented, it probably means the device
> > > should not use any of skb_clone, expand head etc.
> > 
> > Agree. Or maybe force a copy when device uses skb_clone, expand
> > head ...?
> > 
> > Thanks
> > Shirley
> 
> Copy from userspace upfront without locking is probably cheaper? 

Better to prevent this kind of skbs to be used in skb_clone, expand head
for now.

Thanks
Shirley

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