(run 1 — first boot)
$ sh /root/DF-5531/run.sh
== create the usbpf tap clone (root), NO bpf listener attached ==
usbus0: flags=800001<UP,PROXY> metric 0 mtu 0
	groups: usbus
== run the tap-cost rig ==
== cleanup ==
RUN_DONE
$ dmesg | tail -6
DF5531: bus ifp=0xfffff8008fd1e480 if_bpf=0 (expect NULL: no listener)
DF5531: payload=4096 bytes, 20000 iters
DF5531: (A) ifp set,   if_bpf==NULL : 1094 cycles/call
DF5531: (B) ifp NULL (early-return) : 25 cycles/call
DF5531: overhead ratio A/B          : 43.5 x
TAP_RESULT=done