[libvirt] virtio network problems with kvm guests with 2.6.26 kernel

[Jim Paris]

Hi,

After upgrading libvirt on one system, my kvm guests running a 2.6.26
kernel with virtio networking could no longer communicate with the
network.

It seems that the problem is caused by newer libvirt versions
unconditionally enabling GSO support by setting IFF_VNET_HDR [1].

However, support for this feature is apparently broken in 2.6.26.
Ubuntu seems to have also discovered this problem [2] and they
currently work around it by removing GSO support from kvm [3].

Currently, I'm running with a patched libvirt that never sets
IFF_VNET_HDR, and it's working fine once again.

While this is not a libvirt problem per se, it certainly violated the
principle of least surprise given that libvirt was the only thing that
changed!  But I don't know if there's much that libvirt can do to
avoid this problem, as it's really something that needs to get fixed
in kvm, or the guest needs to be upgraded, or switched away from
virtio.

At the very least, maybe this info helps someone else who runs across
the same problem.

-jim

[1] http://www.redhat.com/archives/libvir-list/2009-January/msg00642.html
[2] https://bugs.launchpad.net/kvm/+bug/331128
[3] http://patches.ubuntu.com/k/kvm/extracted/virtio-net_disable_gso.patch