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

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Message-ID: <20131014105527.GD11739@zion.uk.xensource.com>
Date:	Mon, 14 Oct 2013 11:55:27 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	Paul Durrant <Paul.Durrant@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>
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@...rix.com]
> > Sent: 14 October 2013 11:43
> > To: Paul Durrant
> > Cc: xen-devel@...ts.xen.org; netdev@...r.kernel.org; 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.

Wei.
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