netdev - Re: [PATCH net-next 1/2] net: Expose header length compution function

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Date:	Sat, 10 May 2014 10:49:57 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	David Miller <davem@...emloft.net>, amirv@...lanox.com,
	netdev@...r.kernel.org, idos@...lanox.com,
	jeffrey.t.kirsher@...el.com, jesse.brandeburg@...el.com,
	bruce.w.allan@...el.com, carolyn.wyborny@...el.com,
	donald.c.skidmore@...el.com, gregory.v.rose@...el.com,
	alexander.h.duyck@...el.com, john.ronciak@...el.com,
	mitch.a.williams@...el.com, yevgenyp@...lanox.com,
	ogerlitz@...lanox.com
Subject: Re: [PATCH net-next 1/2] net: Expose header length compution
 function

On Sat, 2014-05-10 at 10:12 -0700, Alexander Duyck wrote:

> Actually the flow keys with the exception of maybe thoff and ip_proto
> are pretty much useless to us for our purpose.  As is we are left to
> then go off and compute the header length of the transport before we get
> to the information we need.
> 
> The two functions are very similar though.  It might be worth while to
> actually park the two next to each other so that it is obvious that when
> one gets updated to support a new protocol the other should as well.  So
> for example the headlen function could pick up some of the tunnel stuff.
> 
> The main difference between __net_get_headlen() and skb_flow_dissect()
> is that skb_flow_dissect is meant to work on a packet headed in either
> direction, as such there are multiple calls to skb_copy_bits and all the
> copies out of the paged area that go with it.  __net_get_headlen() is
> meant to be called by the base netdev on a linear buffer to parse
> through everything from the start of the Ethernet header to the start of
> the data section and determine the length of that region.  So one
> obvious difference is that we care about the length of non-l4 headers,
> whereas skb_flow_dissect does not.

Its actually possible to prepare an skb with the needed parts so that
skb_flow_dissect() has no copy to do.

Its a 10 lines helper maybe.

I already mentioned this in the past, I fail to understand your
resistance.

I will provide it unless someone beats me, because its the week end and
I have other plans at the moment.



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