Subject: Re: [PATCHv1 net-next] xen-netback: remove unconditional pull_skb_tail in guest Tx path

From: Ian Campbell <[email-masked]>
Date: Wed, 5 Nov 2014 09:53:05 +0000

On Tue, 2014-11-04 at 16:41 -0500, David Miller wrote:
> From: David Vrabel <[email-masked]>
> Date: Mon, 3 Nov 2014 17:23:51 +0000
> 
> > From: Malcolm Crossley <[email-masked]>
> > 
> > Unconditionally pulling 128 bytes into the linear buffer is not
> > required. Netback has already grant copied up-to 128 bytes from the
> > first slot of a packet into the linear buffer. The first slot normally
> > contain all the IPv4/IPv6 and TCP/UDP headers.
> > 
> > The unconditional pull would often copy frag data unnecessarily.  This
> > is a performance problem when running on a version of Xen where grant
> > unmap avoids TLB flushes for pages which are not accessed.  TLB
> > flushes can now be avoided for > 99% of unmaps (it was 0% before).
> > 
> > Grant unmap TLB flush avoidance will be available in a future version
> > of Xen (probably 4.6).
> > 
> > Signed-off-by: Malcolm Crossley <[email-masked]>
> > Signed-off-by: David Vrabel <[email-masked]>
> 
> Now that this has been discussed a bit, it is possible to get an ack or two?

I'd like to see the commit message expanded to explain why this isn't
introducing a (security) bug by not pulling enough stuff into the header
(IOW the conclusion of the discussion).

Ian.