Comment 0 for bug 1517146

Fast path technology is a user-space stack for high performance packets offloading the Linux networking functions: IPv4/IPv6 routing, linux bridge, iptables, conntrack...

To benefit from the offloading, a VM needs to use vhost-user instead of vhost-net backend for virtio interfaces.

As a consequence, we have:
- a forked ML2 linux bridge mechanism driver: https://github.com/openstack/networking-6wind/blob/master/networking_6wind/ml2_drivers/linuxbridge/mech_driver/mech_lb_fp.py
- a forked linux agent: https://github.com/openstack/networking-6wind/blob/master/networking_6wind/ml2_drivers/linuxbridge/agent/lb_fp_neutron_agent.py.

Problem Description
================

We need to maintain a fork version of the ML2 linux bridge mechanism driver and also one for the linux bridge agent , it is not a proper design.

From users point of view, having to install a specific mechanism driver and an agent to benefit of the fastpath offloading, it is an extra operation. We should avoid to add extra operation for easiness.

Proposed Change
==============

LinuxBridge Agent
--------------------------

The linux bridge agent shall detect whether a fast path offload is enabled, and it shall report it to the mechanism driver with the use of agent_state.

For this capability, a new field will be added in agent_state.configuration: fp_offload.

ML2 LinuxBridge
-----------------------

The mechanism driver LinuxBridge in the try_bind_segment_for_agent will:

- set the vif_type to LINUXBRIDGE if agent['configuration']['fp_offload'] is False
- set the vif_type to VHOSTUSER if agent['configuration']['fp_offload'] is True

Specific vif_details needs to be added for vhost-user:

- vhost_user_socket (i.e '/tmp/usv-XXXX')
- vhost_user_fp_plug should be set True to create a tap netdevice with a vhostuser socket

Note: The vhost_user_fp_plug is a modification under review in Nova. See: https://review.openstack.org/#/c/245369/