I don't really understand why the OS can't just trap the invalid instruction exception and migrate it to the P-core. E.g. AVX-512 and similar. For very old and rare instructions they can emulate them. We used to do that with FPU instructions on non-FPU enabled CPUs way back in the 80s and 90s.

It's not impossible, but it'd be a pain in the butt. If you occasionally use some avx-512 infrequently, no big deal (but also not a big deal to just not use it). But if you use it a lot, all of a sudden your core count shrinks; you might rather run on all cores with avx2. You might even prefer to run avx-512 for cores that can and avx2 for those that can't ... but you need to be able to gather information on what cores support what, and pin your threads so they don't move. If you pull in a library, who knows what they do... lots of libraries assume they can call cpuid at load time and adjust... but now you need that per-thread.

That seems like a lot of change for OS, application, etc. If you run commercial applications, maybe they don't update unless you pay them for an upgrade, and that's a pain, etc.

It's slow and annoying. What would cpuid report? If it says "yes I do AVX-512" then any old code might try to use it and get stuck on the P-cores forever even if it was only using it sparingly. If you say no then the software might never use it, so what was the benefit?

We still do that with some microcontrollers! https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html#index-mf...