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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Nov 05, 2014 at 11:24:16AM +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Ian Campbell
> > Sent: 05 November 2014 11:20
> > To: Paul Durrant
> > Cc: David Vrabel; xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu; Malcolm Crossley
> > Subject: Re: [PATCHv2 net-next] xen-netback: remove unconditional
> > __pskb_pull_tail() in guest Tx path
> > 
> > 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@xxxxxxxxxxxxxxxxxxxx; 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...)
> > 
> 
> Previous versions of the drivers used to parse ARPs so that copies of 
> outgoing gratuitous ARPs could be cached for replay after migrate. Newer 
> drivers acquire IP address bindings from the Windows IP helper (which is now 
> available in kernel) and synthesize ARPs/IPv6 neighbour solicitations.
> 

CC-ing Annie and James - the other two Windows drivers authors..
>   Paul
> 
> > Ian.
> 
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