I feel like this might be a question for @lauriewired Suppose I am running my own local server. I might not want microcode updates that slow down my CPU. The mitigation is a non-issue here. And suppose one CPU is just perfect for my specific case. Market for non-updated CPUs??

microcode get's hot-loaded every boot actually (for x86 at least), it's not persistent to the CPU itself it's the BIOS that loads the patch, and then often the OS on top of that so raw cpu -> bios (patch) -> os (patch again) really, if you want "old" microcode, that means an old BIOS + mitigations=off in the kernel, not an "old" CPU per se. (I think there might be some anti-rollback protections/fuses in some circumstances, but those are still rare-ish)

@lauriewired I don't know why, but I had in my mind that it was an elaborate fuse network. On-boot patching makes a lot more sense. Well now I'm curious about the internals of how a CPU handles patchable instructions. I had the wrong mental model. *off to the reading chair*

@ProgramMax @lauriewired Can't answer for mainstream CPUs, but for a soft-processor on FPGA, it can make sense to only implement core parts of the ISA in hardware; and effectively implement the rest as firmware. Using such an instruction effectively invokes an invisible trap handler, that fakes the op.

@ProgramMax @lauriewired Look into this bughunters.google.com/blog/zen-and-t… It’s a bit lackluster imo

@ProgramMax @lauriewired The CPU microcode is essentially a big lookup table that takes decoder output and turns it into sequence of microinstructions - the fast path ones are simply ones that map to single "line". The microcode mechanism has space to apply patches per instruction

@ProgramMax @lauriewired I don't think it's patching; the "BIOS" just loads the full microcode.

@ProgramMax @lauriewired Budget gaming build YouTubers do this to get extra performance out of old intel cpus, usually rolling back the Meltdown and Spectre patches. Channel called budget builds does it all the time to get defunct hardware performing better