netdev - Re: TX pre-headers...

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Message-Id: <20090209.021403.56830306.davem@davemloft.net>
Date:	Mon, 09 Feb 2009 02:14:03 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	netdev@...r.kernel.org
Subject: Re: TX pre-headers...

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Mon, 9 Feb 2009 21:07:13 +1100

> David Miller <davem@...emloft.net> wrote:
> >
> > I think the way to solve this is to have each device indicate how
> > much TX slack space it neads for it's preheaders.  On device
> > registration we have some global "netdev_max_tx_hdr_space" that
> > records the maximum value seen.
> 
> We can already solve it right now with dev->needed_headroom.
> 
> Of course it's not optimal because it involves a copy so we
> might still need your solution.

Indeed, I noticed needed_headroom when I set out to start coding
things up. :)

There are all sorts of cases that won't work well, as you implicitly
suggest.  For example, forwarding to a path that encapsulates to a GRE
or other type of tunnel.

For IPSEC we're predominantly COW'ing the data anyways, so...
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