7.1.3. time_boot.out
Get a reasonable answer to "how long does boot take in guest time?":
./run --eval-after './linux/time_boot.c'
Source: userland/linux/time_boot.c
That executable writes to dmesg directly through /dev/kmsg a message of type:
[ 2.188242] /path/to/linux-kernel-module-cheat/userland/linux/time_boot.c
which tells us that boot took 2.188242 seconds based on the dmesg timestamp.
Bibliography: https://stackoverflow.com/questions/12683169/measure-time-taken-for-linux-kernel-from-bootup-to-userpace/46517014#46517014