Method 7: return to 16-bit mode, set cs to 0xf000 and jump to 0xfff0.
Linux used to use method 6, but it wasn't reliable either. Something tells me returning to 16-bit mode might not be possible for some hardware / driver combinations.
Tried to get an OS on a PC to soft-reboot, failed, because it is not actually possible to return all hardware to the reset state in some cases - they have an initial power-on state and once programmed can never return there unless hard-reset.
And why does the BIOS require hardware in the hard-reset state? Because a BIOS is never tested in a soft-reset scenarios, because Windows doesn't require it. And "if you haven't tried it, it doesn't work".