arp during live migration

[Cristian Zamfir <zamf@dcs.gla.ac.uk>]

Hi,

I am having some trouble with the send_fake_arp in the netfront driver.

Normally, on my domU, which has no queuing disciplines compiled in, the 
packets are sent via dev_queue_xmit in net/core/dev.c and enqueued using 
pfifo_fast_enqueue in net/sched/sch_generic.c.

However, during live migration, send_fake_arp() returns -2 and does not 
go to pfifo_fast_enqueue any more. I have been able to trace it further 
than this code in dev_queue_xmit:

if (q->enqueue) {
                 /* Grab device queue */
                 spin_lock(&dev->queue_lock);
                 rc = q->enqueue(skb, q);
                 qdisc_run(dev);
                 spin_unlock(&dev->queue_lock);
                 rc = rc == NET_XMIT_BYPASS ? NET_XMIT_SUCCESS : rc;
                 goto out;
          }

I noticed that the error code returned by send_fake_arp() is not 
checked. Would it be a good option to check the error code and 
reschedule the arp broadcast at a later time?

I have made some changes to xen 3.0.3 regarding block device migration 
so I might have messed things up. It could be the reason only few people 
reported this problem on xen-users. Obviously, the problem can also go 
unnoticed if a downtime of 1-2 seconds is tolerated.

Does anyone have any hints on why this might happen or how to search for 
more clues?

Thank you.

Cristian