Re: virtio_net hang

[Mark McLoughlin <markmc@redhat.com>]

On Thu, 2008-11-13 at 13:27 +0100, Emmanuel Lacour wrote:
> Dear kvm users/developpers,
> 
> I have a problem here where the network interface of a guest hang
> 2 or 3 times a day. No more packets can be sent out or received, no
> error in guest or host logs. I have to stop networking, remove module,
> then modprobe again and start the network to get back connection.

The fact that re-loading the virtio_net driver fixes things up makes me
suspect you've found a bug in the virtio_net driver, rather than e.g. a
bug in the kvm-userspace side.

To try and narrow down what's happening, when the interface has hung,
try:

  - tcpdump on both eth0 in the guest and the tap device on the host 
    (tap5 in your example)

  - look for anything unusual in the stats for both those interfaces,
     e.g. /proc/net/dev, netstat -s etc.

  - strace the /usr/bin/kvm process

What you're looking for e.g. is whether a guest->host ping is failing
because the request packet isn't getting to the host or because the
reply packet isn't getting to the guest, and where exactly the packet is
being blocked.

Cheers,
Mark.