Re: [PATCHv2 net-next] xen-netback: remove unconditional __pskb_pull_tail() in guest Tx path

[Ian Campbell <Ian.Campbell@citrix.com>]

On Wed, 2014-11-05 at 11:17 +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Ian Campbell
> > Sent: 05 November 2014 11:00
> > To: David Vrabel
> > Cc: xen-devel@lists.xenproject.org; Wei Liu; Malcolm Crossley; Paul Durrant
> > Subject: Re: [PATCHv2 net-next] xen-netback: remove unconditional
> > __pskb_pull_tail() in guest Tx path
> > 
> > Dropping netdev since this isn't relevant to them, adding Paul
> > 
> > On Wed, 2014-11-05 at 10:50 +0000, David Vrabel wrote:
> > > - performance: Netback has already grant copied up-to 128 bytes from
> > >   the first slot of a packet into the linear area. The first slot
> > >   normally contain all the IPv4/IPv6 and TCP/UDP headers.
> > 
> > Does "normally" include guests other than Linux? I thought Windows in
> > particular was prone to splitting the headers into a frag per layer or
> > thereabouts?
> > 
> 
> Current upstream Windows PV drivers will put all parsed headers in the
> first frag and the rest of the packet in subsequent flags. The parser
> currently knows about TCP and UDP over IPv4 or v6, with and without
> SNAP encapsulation. It doesn't, for example, know about ARP so the
> backend will see only the ethernet header in the first frag.

Sounds like that is sufficient to reach the "normally" qualification,
thanks.

(I wonder what sort of benefit parsing arp would bring...)

Ian.