[Qemu-devel] bringing up virtio network device hangs guest

[Stuart Yoder]

I'm trying to solve a problem where bringing up a virtio network
device in a KVM guest hangs the guest.

Start QEMU with these options:
   -net nic,model=virtio -net tap,script=/root/qemu-ifup

The qemu-ifup script is pretty simple, just adds the interface passed
in to a bridge:
  #!/bin/sh
  bridge=br0
  guest_device=$1
  ifconfig $guest_device up
  brctl addif $bridge $guest_device

The guest kernel finds the virtio device, but when bringing up the
virtio eth0 device in the
guest the ifconfig hangs the guest.

In the host, the bridge (br0) is able to send and receive packets.

The tap device created by QEMU shows packets sent, but none received:

tap0      Link encap:Ethernet  HWaddr 0A:02:56:5A:72:C6
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:257 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:500
          RX bytes:0 (0.0 B)  TX bytes:17592 (17.1 KiB)

Wondered if anyone had any ideas on how to further diagnose what might
be going on.

Thanks,
Stuart