[pedrocr]

If the kernel itself doesn't boot, installing the microcode package may not help. It only helps if the problem only occurs after the point at which the kernel loads the microcode. Hopefully that's early enough for most cases.

[nemetroid]

Early load microcode updating happens very early for this precise reason.

[throwaway8941]

Indeed. At least the Intel driver does it so early in the boot process that the "microcode updated" message is literally the first line in dmesg on a couple of systems I've bothered to check.