netdev - Re: [PATCH V3 2/8] Add a new zerocopy device flag

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Message-ID: <1304444570.20660.32.camel@localhost.localdomain>
Date:	Tue, 03 May 2011 10:42:50 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	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 V3 2/8] Add a new zerocopy device flag

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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