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Comment by pantalaimon | Hacker News Reader

Since Linux loads the microcode on it's own, can this be used to apply an old microcode or is there a prevention against downgrading the version?

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I'm pretty sure downgrades are blocked. You need to hack the BIOS so the (volatile) upgrade never happens on boot.

Microcode updates are applied at runtime, every time you boot them. At least on the CPU side. The motherboard might apply them before the OS loads though, so who knows what happens there

Thats what I said; the updates are volatile but the OS can't undo an update that the BIOS already applied on boot.

I think for the subset of people that care about having AVX512 on their 12900K, and bother to disable the E-cores to achieve it, they will be using enthusiast motherboards that don't block bios upgrades in this way.

Microcode downgrades are blocked by the CPU itself; the BIOS doesn't get a say. You could downgrade the whole BIOS to stop the upgrade being applied on boot (maybe, some motherboards may try to block this too), but then you need to choose between uncrippled microcode and BIOS bugfixes and improvements.

I wouldnt call it hacking. If you can find or backup the bios image for your motherboard, you can replace the microcode in it and reflash. Bios images are modular and the microcode is a replacable component.

Hacking BIOS images isn't always that easy. Often they are signed (EFI Capsules) and the standard update utility in the BIOS menu will reject modified versions. I had to use a convoluted flashing process to make a trivial patch to a BIOS a few years back for this reason.

You mean, SOIC clip + flashrom?

That usually works, yes (modulo Boot Guard and electrical issues), but we're getting into less accessible methods, aren't we :)

In my case I managed to get it to work with some random vendor tools run from DOS, which bypassed the EFI flash subsystem. Definitely not a pleasant experience.

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