Today, when an instance sends an ICMPv6 Neighbor Solicitation (NS) 
packet, it gets sent to ovn-controller which will reply with the 
appropriate NA.

While this works, it doesn't match the IPv4 behavior for ARP where 
all the actions are implemented at OVS level without requiring the 
controller to be up.

This not only seems to create a dependency for the IPv6 dataplane on 
the controlplane (ovn-controller) but I assume that, depending on the 
traffic pattern, there could also be a performance hit as all these 
actions will be processed by a single thread in ovn-controller.

It looks like this has been already requested upstream in the past [0] and there was some limitations but they seem to have been resolved already at userspace [1].

Not sure about kernel level though.

I'll defer to the core OVN team to assess whether this is technically
 possible at the moment or we need additional RFEs that target different
 components.

[0] https://mail.openvswitch.org/pipermail/ovs-discuss/2020-July/050388.html

[1] http://patchwork.ozlabs.org/project/openvswitch/patch/20200928134947.48269-1-fankaixi.li@bytedance.com/