Comment 3 for bug 584048

Probably network freeze occur because of adding (on guest start-up) created tun/tap interface to the bridge.
On guest shutdown tun/tap interface are removed from bridge. This add/remove operation to/from bridge of tap interface causes network freeze in my case. I have play around by replacing '-net tap' to '-net tap,ifname=tap.0,script=no,downscript=no' - no freeze occur. But in this case guest are not connected to the outside world.

I have no idea does network freeze problem related to the virtual network kernel drivers or is it related to the physical network interface drivers of the host machine (in my case host if is 'RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 02)').

Also tested following, run ping:
from LAN to IP of bridged if - time out during freeze
from bridged machine to bridged if - no freeze
from bridged machine to LAN - time out during freeze

I have tested on:
Lucid x64 + latest updates
installed eucalyptus cloud
kernel: 2.6.32-22-server
qemu-kvm: 0.12.3+noroms-0ubuntu9