Userspace sure, but what about underlying hardware? How will device drivers react if they come up and encounter hardware that is not coming out of a ACPI induced reboot? Will some devices and their corresponding drivers be OK? Or will the drivers panic when they encounter a device in a weird state?
I'm genuinely curious is all. At the time I was pursing this I decided it was going to get too complicated and that I had to live with a reboot.
This is a concern but it usually works for two reasons:
1. Most firmware is sufficiently broken that Linux drivers are already hardened against devices being brought up in arbitrary states.
2. kexec walks the device tree to shut down all devices before starting the new kernel. This usually gets devices closer to a startup state, or at least a smaller number of known shutdown states.
