Re: regression: UFO removal breaks kvm live migration

On Tue, Nov 7, 2017 at 5:02 PM, Michal Kubecek [off-list ref] wrote:

Hello,

I just received this bug report:

  https://bugzilla.suse.com/show_bug.cgi?id=1066757

The reporter runs a live migration of a kvm guest from a host with
kernel supporting UFO (openSUSE 42.2 or 42.3, based on 4.4) to a host
with kernel with UFO support removed (SLE15 or openSUSE 15.0 pre-release
which is based on 4.12 but has the UFO removal series backported).

The migration fails with

  kvm: virtio-net: saved image requires TUN_F_UFO support

because the guest image has a virtio_net device with UFO enabled which
requires TUN_F_UFO on the corresponding host tun device but that is no
longer available on the target host.

This kind of problem already happened once:

  https://www.spinics.net/lists/netdev/msg443821.html

At that time, commit 3d0ad09412ff ("drivers/net: Disable UFO through
virtio") was reverted once the issue it worked around was resolved in
a different way.

I didn't have time to think it through yet but perhaps we could allow
setting TUN_F_UFO and ignore its value.

If the feature is enabled guests may try to send UFO packets, which
the host is no longer able to fragment.

virtio_net_hdr_to_skb will drop the packets immediately based on
gso_type and tun_get_user will return EINVAL.

Still, perhaps that's preferable as migration will succeed and most
guests won't ever try to send those packets in the first place.

This is not time critical for SLE15 / openSUSE 15.0 which are still at
early beta stage but 4.14 final is close and once it's out, more users
are going to hit this.

Michal Kubecek