Re: [PATCH 1/2] NET: Re-add VLAN tag for devices incapable of keeping it

[David Miller <davem@davemloft.net>]

From: Patrick McHardy <kaber@trash.net>
Date: Tue, 06 Nov 2007 01:21:09 +0100

> David Miller wrote:
> > From: Patrick McHardy <kaber@trash.net>
> > Date: Mon, 05 Nov 2007 19:00:19 +0100
> > 
> >> This looks like a rather expensive operation for the unlikely case
> >> that packets will be received by a packet socket. IMO it should only
> >> be reconstructed if actually needed, by af_packet itself.
> > 
> > Completely agreed.  We should not do this by default when %99
> > of the networking stack simply does not care about this.
> 
> 
> I think there is one more case that matters, which is briding
> from a device with VLAN stripping for a VLAN not configured
> locally. The tag will be stripped and will be lost for forwarded
> packets. But I'm not exactly sure this really can be configured
> (time for bed so I'll check tommorrow).

If so then when such rules are loaded, just like PF_PACKET, it can set
the indication to start reconstituting VLAN headers stripped by HW.

> >> As we discussed some time back storing the VLAN tag in the CB on
> >> TX clashes with other users of the CB like qdiscs, so we need a
> >> new field in the skb for this anyway.
> > 
> > Someone will have to find a way to remove some other fields in
> > sk_buff before I'm going to allow more space to be eaten up
> > by this completely fringe case feature.
> 
> We have a two byte hole after tc_verd where we could fit this in.
> But I'm pretty sure we also could reuse some other fields on input,
> like queue_mapping or maybe even destructor for unowned skbs.

Two bytes is enough, so if there is a hole we can use it.
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