Re: xen2 and VLANs (again)

[Greg Troxel <gdt%ir.bbn.com@localhost>]

Manuel Bouyer <bouyer%antioche.eu.org@localhost> writes:

> On Tue, Dec 11, 2007 at 10:47:16PM -0600, Chris Brookes wrote:
>> On 11/12/2007, Manuel Bouyer <bouyer%antioche.eu.org@localhost> wrote:
>> 
>> > Yes, it's the way to go
>> 
>> Sure enough it works  as planned. I'm very happy with the setup.
>> 
>> > From a quick look at current, it looks like the MTU is hardwired in
>> > the vr(4) driver. But if you have a patch from openbsd, it shouldn't
>> > be hard to get it working.
>> 
>> 
>> I appeared to just need
>> 
>> sc->vr_ec.ec_capabilities = ETHERCAP_VLAN_MTU;
>> tmp = ifp->if_mtu + ETHER_HDR_LEN + ETHER_CRC_LEN;
>> tmp += ETHER_VLAN_ENCAP_LEN;
>> ifp->if_mtu = tmp;
>> 
>> And it seems to work well. I hesitate to submit a patch because I have
>> very little confidence in what I've done, particularly since I've only
>> had it running for 10 minutes
>
> please send a PR with patch when you have tested it a bit more. Especially,
> test (with e.g. ping -s) that you can send 1500-bytes packets.

Why is it correct to add ETHER_HDR_LEN and ETHER_CRC_LEN?  I would say
that comments arguing that the logic is correct are needed too (but
maybe I just don't get something).