From: DaanDeMeyer on gitlab.com https://gitlab.com/cki-project/kernel-ark/-/merge_requests/2817#note_1695011...
Note that with a systemd based initrd without dracut it's sufficient to add "root=<tag> rootfstype=virtiofs rw" to boot from virtiofs (with or without initrd if virtiofs is builtin). I haven't had to add anything from dracut to the initrd to get this working in mkosi. But I guess in this case the virtiofs module is at least required to pull in the kernel module. Not sure what else it does.