> A colleague is working on a kernel patch to bypass part of the UDP stack to restore proper ordering, which will probably need to be maintained in-house or at least hidden behind a kernel config knob upstream since this reordering is considered acceptable under the guarantees for UDP.

My understanding is that in Linux, TCP on localhost (including packets sent to a public address on a local interface) bypasses the stack; I don't see why it would be a problem for UDP to do the same.

This is contrast with FreeBSD, where TCP to localhost is actually packetized and queued on the loopback interface, and you can experience congestion collapse on loopback if you do things right(wrong).