Re: [Xen-devel] [PATCH net-next v4 2/5] xen-netback: add support for IPv6 checksum offload from guest

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> Sent: 14 October 2013 11:55
> To: Paul Durrant
> Cc: Wei Liu; xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; David Vrabel;
> Ian Campbell
> Subject: Re: [PATCH net-next v4 2/5] xen-netback: add support for IPv6
> checksum offload from guest
> 
> On Mon, Oct 14, 2013 at 11:49:20AM +0100, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > > Sent: 14 October 2013 11:43
> > > To: Paul Durrant
> > > Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Wei Liu; David
> Vrabel;
> > > Ian Campbell
> > > Subject: Re: [PATCH net-next v4 2/5] xen-netback: add support for IPv6
> > > checksum offload from guest
> > >
> > > On Fri, Oct 11, 2013 at 04:06:19PM +0100, Paul Durrant wrote:
> > > [...]
> > > > -/*
> > > > - * This is the amount of packet we copy rather than map, so that the
> > > > - * guest can't fiddle with the contents of the headers while we do
> > > > - * packet processing on them (netfilter, routing, etc).
> > > > +/* This is a miniumum size for the linear area to avoid lots of
> > > > + * calls to __pskb_pull_tail() as we set up checksum offsets.
> > > >   */
> > >
> > > You seem to forget to explain why 128 is chosen. :-)
> >
> > Is that not sufficient explanation? What sort of thing are you looking for?
> >
> 
> From the second version of this patch, we had a conversation.
> 
> > Where does 128 come from?
> >
> 
> "It's just an arbitrary power of 2 that was chosen because it seems to
> cover most likely v6 headers and all v4 headers."
> 
> So something like: "We choose 128 which is likely to cover most V6
> headers and all V4 headers" would be sufficeint.
> 

Ok. I figured that was implied by "set up checksum offsets" but I can be more 
explicit.

  Paul

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