Macvtap networking for qemu/kvm/libvirt VMs

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tldr; How do I use macvtap for VMs on NixOS?

Tailscale support advised me to use macvtap networking in libvirt to allow VMs to bypass the Tailscale exit node. But because it gets into platform-specific configuration, they aren’t telling me how. I’ve never used macvtap before, and I’m trying to figure out how to implement this in NixOs.

I found these general articles about macvtap:

I got as far as apparently confirming I have support for macvtap available on this machine, but I don’t know where to go from here:

[:~]$ lsmod | grep macvlan
macvlan                36864  0

This page has a a tip on using macvtap on NixOS, but not how to configure it.
https://nixos.wiki/wiki/Using_bridges_under_NixOS

The other pages on NixOS and libvirt do not address macvtap, or at least not in a way that I’m seeing.
https://nixos.wiki/wiki/Libvirt
https://nixos.wiki/wiki/Virt-manager

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In case anyone else stumbles across this, the instructions from the general articles about macvtap linked above worked for me. There might be a more Nixy way to do it, but I didn’t go down that path. It worked, so I’m good.