Comment 7 for bug 1742505

Simple reproducer:

sudo add-apt-repository cloud-archive:pike
sudo apt update
sudo apt install openvswitch-switch
sudo ovs-vsctl add-br br-tun
sudo ovs-vsctl add-port br-tun gre0 -- set interface gre0 type=gre options:remote_ip=10.100.1.1

this will cause ovs to create the gre_sys device in the kernel; the remote_ip can be anything but is obviously not functional.