netdev - Re: [patch net-next-2.6 v2] net: vlan: make non-hw-accel rx path similar to hw-accel

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Date:	Sun, 22 May 2011 11:20:09 +0200
From:	Michał Mirosław <mirqus@...il.com>
To:	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>
Cc:	Jiri Pirko <jpirko@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jesse Gross <jesse@...ira.com>,
	Changli Gao <xiaosuo@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	shemminger@...ux-foundation.org, kaber@...sh.net, fubar@...ibm.com,
	eric.dumazet@...il.com, andy@...yhouse.net
Subject: Re: [patch net-next-2.6 v2] net: vlan: make non-hw-accel rx path
 similar to hw-accel

W dniu 22 maja 2011 11:10 użytkownik Nicolas de Pesloüan
<nicolas.2p.debian@...il.com> napisał:
> Le 22/05/2011 10:52, Michał Mirosław a écrit :
>>
>> 2011/5/22 Nicolas de Pesloüan<nicolas.2p.debian@...il.com>:
>>>
>>> Le 22/05/2011 08:34, Eric W. Biederman a écrit :
>>>>
>>>> Jiri Pirko<jpirko@...hat.com>    writes:
>>>>
>>>>> Sun, May 22, 2011 at 04:59:49AM CEST, nicolas.2p.debian@...il.com
>>>>> wrote:
>>>>>
>>>>> <snip>
>>>>>>
>>>>>> And because some setups may still require the skb not to be untagged,
>>>>>> may be we need the ability to re-tag the skb in some situations...
>>>>>> When a protocol handler or rx_handler is explicitly registered on a
>>>>>> net_device which expect to receive tagged skb, we should deliver
>>>>>> tagged skb to it... Arguably, this may sound incredible for the
>>>>>> general case, but may be required for not-so-special cases like
>>>>>> bridge or protocol analyzer.
>>>>>
>>>>> Wait, what setups/code require the skb not to be untagged? If there's
>>>>> such, it should be fixed.
>>>>
>>>> tcpdump on the non-vlan interface for one.
>>>
>>> bridge is another. More precisely, there is a difference between the
>>> following two setups:
>>>
>>> 1/ eth0 - eth0.100 - br0 - eth1.200 - eth1
>>>
>>> 2/ eth0 - br0 - eth1
>>>
>>> In case 1, it is normal and desirable for the bridge to see untagged skb.
>>>
>>> In case 2, it is desirable for the bridge to see untouched (possibly
>>> tagged)
>>> skb. If current bridge implementation is able to handle skb from which we
>>> removed a tag, in this situation, it means that bridge currently "fix
>>> improper untagging" by itself, by forcing re-tagging on output. I think
>>> is
>>> should not be the job of protocol handlers to fix this. Again, a generic
>>> feature should to it when necessary.
>>>
>>> Think of the following setups:
>>>
>>> 3/ eth0 - br0 - eth1.200 - eth1.
>>> 4/ eth0 - eth0.100 - br0 - eth1
>>>
>>> What if one expect this setup to add (3) or remove (4) one level of vlan
>>> nesting? This is precisely what this setup suggest. How can we instruct
>>> the
>>> bridge to do so? It is not the bridge responsibility to do any vlan
>>> processing. bridge is expected to... bridge !
>>
>> I assumed that this untaging Jiri is implementing does not remove the
>> tag. It moves the information from skb->data to skb->vlan_tci, but the
>> information contained is not otherwise changing. All your examples
>> should work regardless of where the tag is stored.
>
> I assumed (but didn't tested) that this untagging also change the starting
> point of the payload of the packet. So protocol handlers expecting to have
> the raw packet won't see the vlan header.

That would also be the case with hardware stripped tags - they need to
look into skb->vlan_tci anyway.

Best Regards,
Michał Mirosław
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