Was playing with #kexec and getting a hang/blank screen when attempting to reboot.
Guess what: the only person on the intertubes that also has this problem is another #Guix user:
https://unix.stackexchange.com/questions/781898/how-to-use-kexec-on-guix-getting-a-blank-screen
Debugging tips welcome!
Turns out you have to do ‘kexec --type=bzImage64 -l /run/current-system/kernel/bzImage …’.
What does this ‘--type’ option does? Quite a few important things, it seems!
Which means that ‘kexec_file_load’ is deceptive: it looks like it’ll take file descriptors and do its job, but apparently no, something non-trivial has to be done beforehand (as can be seen in the kexec-tools code).
In the end, ‘kexec_file_load’ works as advertised, problem was between keyboard and chair:
https://issues.guix.gnu.org/75027
(‘kexec-tools’ does contain a whole bunch of code for the older ‘kexec_load’ syscall though, and that contributed to my confusion.)
Ludovic Courtès
🗃️system loaded for fast reboot in PATCH 3/3, commenting here since I'm away from email environment till next weekend 
Janneke
Efraim Flashner