netdev - regression: UFO removal breaks kvm live migration

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Message-ID: <20171107080224.v6z65jvimpa5ohs4@unicorn.suse.cz>
Date:   Tue, 7 Nov 2017 09:02:24 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org
Subject: regression: UFO removal breaks kvm live migration

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.

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