[Bug 1742505] Re: gre_sys set to default 1472 when using path_mtu > 1500 with ovs 2.8.x

[James Page]

I've re-tested with the proposed changes upstream; MTU is correctly set
to 65000 for all tunnel types including GRE on 4.4 kernels; the kernel
fix for bug 1743746 is required for support on >= Artful or if the HWE
edge kernel is in use on Xenial.

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https://bugs.launchpad.net/bugs/1742505

Title:
  gre_sys set to default 1472 when using path_mtu > 1500 with ovs 2.8.x

Status in Ubuntu Cloud Archive:
  In Progress
Status in Ubuntu Cloud Archive pike series:
  In Progress
Status in Ubuntu Cloud Archive queens series:
  In Progress
Status in neutron:
  Invalid
Status in linux package in Ubuntu:
  Confirmed
Status in openvswitch package in Ubuntu:
  In Progress
Status in linux source package in Artful:
  Confirmed
Status in openvswitch source package in Artful:
  In Progress
Status in linux source package in Bionic:
  Confirmed
Status in openvswitch source package in Bionic:
  In Progress

Bug description:
  Setup:
  Pike neutron 11.0.2-0ubuntu1.1~cloud0
  OVS 2.8.0
  Jumbo frames setttings per: https://docs.openstack.org/mitaka/networking-guide/config-mtu.html
  global_physnet_mtu = 9000
  path_mtu = 9000

  Symptoms:
  gre_sys MTU is 1472
  Instances with MTUs > 1500 fail to communicate across GRE

  Temporary Workaround:
  ifconfig gre_sys MTU 9000
  Note: When ovs rebuilds tunnels, such as on a restart, gre_sys MTU is set back to default 1472.

  Note: downgrading from OVS 2.8.0 to 2.6.1 resolves the issue.

  Previous behavior:
  With Ocata or Pike and OVS 2.6.x
  gre_sys MTU defaults to 65490
  It remains at 65490 through restarts.

  This may be related to some combination of the following changes in OVS which seem to imply MTUs must be set in the ovs database for tunnel interfaces and patches:
  https://github.com/openvswitch/ovs/commit/8c319e8b73032e06c7dd1832b3b31f8a1189dcd1
  https://github.com/openvswitch/ovs/commit/3a414a0a4f1901ba015ec80b917b9fb206f3c74f
  https://github.com/openvswitch/ovs/blob/6355db7f447c8e83efbd4971cca9265f5e0c8531/datapath/vport-internal_dev.c#L186

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