netdev - Question about more headroom in skb

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Date:	Mon, 10 May 2010 13:09:27 -0700 (PDT)
From:	Sharat Masetty <sharat_masetty@...oo.com>
To:	netdev@...r.kernel.org
Subject: Question about more headroom in skb

Hello All,

For my project I need 3 words of headroom in the skb in the network driver level, to add a  custom header to the ethernet packet. I looked into the tcp code and figured out tcp uses sk->sk_prot->max_header for header allocation size. But I was not able to confirm that all other transport protocol use the same mechanism(?) For example in UDP/ICMP I was not able to figure out from the code where the allocation and header reservation happens(Any light here would be really helpful.)

I have also looked at an API in skbuff skb_pad() which does what I want(add either headroom or tailroom), but I want to avoid that for performance reasons(skb_pad does kmalloc and memcpy). I want to figure out a good way(may be tune some parameters) to allocate extra 3 words for any skbuff independant of the transport protocol being used. Any light here would be very much appreciated.

Thanks,
Sharat.

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