(df5531_tapcost KLD built on guest; from live session)
$ cd /root/DF-5531 && make
awk -f dragonfly/tools/makeobjops.awk -- -h dragonfly/kern/bus_if.m
awk -f dragonfly/tools/makeobjops.awk -- -h dragonfly/kern/device_if.m
cc  -O2 -pipe -D_KERNEL -Wall ... -Werror -DKLD_MODULE -nostdinc -c tap_cost.c
cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv -r -Wl,-d -o df5531_tapcost.ko tap_cost.o
ls -l df5531_tapcost.ko
-rw-r--r-- 1 root wheel 6264 Oct  6 23:53 df5531_tapcost.ko
BUILD_OK