rename init_dev_kmsg to time_boot and move doc to README

Author: cirosantilli

README.adoc

@@ -2363,6 +2363,24 @@ Source: link:kernel_module/user/sleep_forever.c[]
 
 This executable is a convenient simple init that does not panic and sleeps instead.
 
+==== time_boot.out
+
+Get a reasonable answer to "how long does boot take?":
+
+....
+./run -F '/time_boot.out'
+....
+
+Dmesg contains a message of type:
+
+....
+[    2.188242] time_boot.c
+....
+
+which tells us that boot took `2.188242` seconds.
+
+Bibliography: https://stackoverflow.com/questions/12683169/measure-time-taken-for-linux-kernel-from-bootup-to-userpace/46517014#46517014
+
 [[init-busybox]]
 === Run command at the end of BusyBox init
 

kernel_module/user/init_dev_kmsg.c renamed to kernel_module/user/time_boot.c

@@ -1,4 +1,4 @@
-/* https://stackoverflow.com/questions/12683169/measure-time-taken-for-linux-kernel-from-bootup-to-userpace/46517014#46517014 */
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#time_boot-out */
 
 #include <stdio.h>
 #include <unistd.h>