Comment 16 for bug 1815989

no we cant activate teh port biding before the vm is runing on the the dest unless you are using post copy live migration.

if you are using post copy live migrate we recive an event form libvirt when the vm is pasused and we activate the port binding at that point which is the first point where we can safely activate the port.

when we activate teh port on the destiatnion the port binding on the source is deactivated and in theory the network backend should prevent any network traffic form been sent or received on the source node. meaning if the ovs agent was correctly isolating the interface when it was deactivated then you would have a netwrok partation if we just activated it before we migrate which is why we do not do that. we cannot start the vm on the dest before stoping it on the souce node as that is entirely managed out side of nova control by libvirt.

if you are not using post copy we activate teh bining in the same place we woudl ahve previously bound the port in post_livemigration.

you are correct that we still have fallback code for neutron version that dont support multiple port bindign for live migration that takes the old path. if hardcoding the check to false fixes the problem it impleies the neutron_l2_agent is incorrectly wiring up ports on hosts that the port is not bound too which is not correct behaviour.

https://review.opendev.org/#/c/640258 allows neutron to wire up the destination port before the vm is created on the destination. https://review.opendev.org/#/c/602432/ was intended to prevent the race between libvirt and the l2 agent.

that is the only way i know of too fix this currently.