Once per process. I doubt operating systems will start modifying executables and libraries on-disk to tag them as using relevant instruction set extensions.

And generally speaking, when an application first starts issuing SIMD instructions, that's probably not the a great time to be interrupting it even if it only needs to happen once.

once per process is perfectly fine.

the issue is that once you have an active process using a feature only available on the larger cores, you can't shut off the larger cores to save power without paying a large latency to wake up that process.

You could imagine it as an ELF note of some kind. It's not a direction I've love to go in, and catching illegal exception traps seems fast enough that the more complicated design doesn't seem worthwhile.

Well, once when it's about to try to wring maximum performance out of the processor, and if you only do it once you're ensured that your smaller cores never get used…