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: David Vrabel
> Sent: 14 October 2013 13:19
> To: Wei Liu
> Cc: Paul Durrant; netdev@xxxxxxxxxxxxxxx; Ian Campbell; David Vrabel; xen-
> devel@xxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH net-next v4 2/5] xen-netback: add support
> for IPv6 checksum offload from guest
> 
> On 14/10/13 11:55, Wei Liu wrote:
> > 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.
> 
> Is "most IPv6 headers" actually good enough?  Don't we need to ensure
> netback copies all IP headers?
> 

It will do if checksum offload is in use, but perhaps the pull as far as the 
transport header needs to be done anyway? I'm unsure of the expectations of 
other code.

  Paul

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