netdev - Re: skb header allocation

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Message-ID: <4A94CFB3.4020706@linux.vnet.ibm.com>
Date:	Wed, 26 Aug 2009 08:01:23 +0200
From:	Frank Blaschka <blaschka@...ux.vnet.ibm.com>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	Chris Ross <chris@...pilednetworks.com>, netdev@...r.kernel.org
Subject: Re: skb header allocation

Is it save to change dev->hard_header_len to a value other than ETH_HLEN for
an ethernet device?

I rememeber we run in trouble with some kernel components (netfilter?, I'm
not sure can't remember the details ...) when we did this is the past.

Is this not an issue any more?

Thanks

Stephen Hemminger schrieb:
> On Tue, 25 Aug 2009 22:04:10 -0500
> Chris Ross <chris@...pilednetworks.com> wrote:
> 
>> I have a network driver that acts as a Ethernet device and builds up a
>> series of outer headers on skb(s) it receives from upper layers. I am
>> currently using the technique that is in ipip.c to ensure I have
>> enough room to add my header ...
>>
>> if (skb_headroom(skb) < some_value || skb_shared(skb) ||
>>          ((skb_cloned(skb) && !skb_clone_writable(skb, 0))))
>>    {
>>       if ((skb2 = skb_realloc_headroom(skb, some_value)) == NULL)
>>          return -1;
>>
>>       dev_kfree_skb(skb);
>>       skb = skb2;
>>    }
>>
>> Is this the best practice for a high bandwidth scenario?
> 
> Define a value of dev->hard_header_len that adds space for what you need.
> Use skb_cow_head(skb, headroom) before touching skb header.
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