Re: [Xen-users] Ethernet MTU

[Dominique Rousseau <d.rousseau@xxxxxxx>]

Le Wed, Aug 16, 2006 at 02:09:08PM +0200, Dominique Rousseau 
[d.rousseau@xxxxxxx] a écrit:
> Le Wed, Aug 16, 2006 at 12:13:35PM +0200, Sylvain Coutant [sco@xxxxxxxxxx] a 
> écrit:
> > Hi,
> > 
> > Is there any plan to raise supported ethernet mtu above 1500 ?
> > Currently, packets are dropped above this point (tested against
> > xen-3.0.2-testing). Even 802.1q tags require explicitly lowering the
> > mtu to 1496 to work.
[...]
> Never tried/looked if Xen's vif layer put a limitation on this,
> although.
> (which should be quite trivial to patch, if any)

After some tries, it looks like the vif code don't handle frames
with size larger than 1500.
Looking at the code, there is the following snippet that seems to be
the probleme (in drivers/xen/netback/netback.c) :

               if (unlikely(txreq.size < ETH_HLEN) || 
                   unlikely(txreq.size > ETH_FRAME_LEN)) {
                       DPRINTK("Bad packet size: %d\n", txreq.size);
                       make_tx_response(netif, txreq.id, NETIF_RSP_ERROR);
                       netif_put(netif);
                       continue; 
               }

If I get some time tomorrow, I'll try increasing it to see if it's correcting
the problem.


Dominique

-- 
Dominique Rousseau 
Neuronnexion, Prestataire Internet & Intranet
57, route de Paris 80000 Amiens
tel: 03 22 71 61 90 - fax: 03 22 71 61 99 - http://www.neuronnexion.fr

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