netdev - Re: [PATCH net-next V2] gro: introduce gro_mac_header

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Date:	Mon, 06 Feb 2012 14:56:52 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	roland@...nel.org
Cc:	eric.dumazet@...il.com, ogerlitz@...lanox.com,
	sean.hefty@...el.com, herbert@...dor.hengli.com.au,
	linux-rdma@...r.kernel.org, shlomop@...lanox.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next V2] gro: introduce gro_mac_header_len

From: David Miller <davem@...emloft.net>
Date: Mon, 06 Feb 2012 14:51:48 -0500 (EST)

> From: David Miller <davem@...emloft.net>
> Date: Mon, 06 Feb 2012 14:32:30 -0500 (EST)
> 
>> From: Roland Dreier <roland@...nel.org>
>> Date: Mon, 6 Feb 2012 11:23:21 -0800
>> 
>>> Does the netdev driver own skb->cb between hard_header
>>> and start_xmit?  If so we could use that instead of stealing
>>> some header space, and that would at least let us not lie
>>> about hard_header_len.
>> 
>> Unfortunately the packet scheduler sits between the hard_header()
>> call (via neigh_*_output() --> dev_hard_header()) and when the
>> device xmit method is invoked.
>> 
>> And the packet scheduler can make use of the skb->cb[], for
>> include/net/sch_generic.h:qdisc_skb_cb
> 
> Actually there is a way to make this work.
> 
> Define your ipoib_skb_cb something like:
> 
> struct ipoib_skb_cb {
> 	struct qdisc_skb_cb	qdisc_cb;
> 
> 	... ipoib stuff goes here ...
> };
> 
> That way you can use the SKB cb area for your ipoib info
> without interfering with the packet scheduler.

But this needs a little bit of work since the qdisc_skb_cb ends with a
variable length array, but we can put an upper bound on this just like
we do for skb->cb[] itself to fix this issue.

I'll toss something together.
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