if there is no reason for them to be different we could consider remvoing the supprot for that in nova but that may have upgrade impacts it woudl be better to update the mech driver to have it set the same value in both places first and then update nova in a later release.
We'll still need to test this but reading the comments in https://gitlab.com/libvirt/libvirt/-/blob/v10.0.0/src/qemu/qemu_interface.c#... I don't think so, the mac addresses have to be different if this is true. In addition, given this "increase in latency" issue exists, my guess is that the packets will include the hyp-side and VM-side mac addresses, because hypervisor is dropping them when VM uses the wrong mac address. If this is the case, then if we have the same mac address on both the VM side and hyp side, then I would expect errors like "received packet on vnetX with own address as source address" come up. So they probably still need to be different.