On Monday, 12 April 2021 11:56:40 BST Adam Carter wrote:
> Do these largely overlap?

Yes.

> So if your motherboard manufacturer is diligent with releasing updates and
> you've applied them, you generally won't expect a 'microcode updated early
> to new patch_level' message from dmesg?

From what I've come across so far, it's been the case MoBo manufacturers 
release BIOS/UEFI code upgrades for a few years only.  Dekstop MoBos up to 5 
years after product launch, laptops up to 2 years if you're lucky.  Thereafter 
CPU microcode patching becomes an exercise for the OS.  However, the CPU 
manufacturer may not bother releasing updates for your CPU model, in which 
case the result is the same.  :(

The signature of the microcode will change after an OEM BIOS/UEFI code upgrade 
has been flashed into the MoBo, so you'll be able to tell if the upgrade had 
anything to do with the CPU.