I'm glad to see this. Almost 18 years ago I implemented a similar kexec device+memory preservation for a storage vendor. It was done on a Linux kernel of that day, and it had had a memory reservation and handoff protocol between the two kernels to keep some specific PCI device alive, allowing for state restoration at the application side. I'm proud of the fact that the kernel replacement was just under 1 second in execution (after init process optimization) and the whole kernel+app was less than 10 seconds.

There's also `systemctl soft-reboot` which initiates a userspace-only reboot, which quickly restarts the system without going through the full hardware and kernel initialization process.

Was hoping this would be mentioned. This version of functionality will probably be more universally accessible to all configurations as compared to the soft reboot released in 254:

https://github.com/systemd/systemd/releases/tag/v254

Soft rebooting was even more exciting for me, but I wasn't able to get it to work on my bluebuild based system (customized universal blue). I haven't tried it in a while though.