> AVX512 isn't available on most new CPUs
Please define new. Also, I think AMD uses very similar cores in server and client. So, disabling AVX512 may be an Intel thing (my guess is that so they can easily move threads between E & P cores).
Please define new. Also, I think AMD uses very similar cores in server and client. So, disabling AVX512 may be an Intel thing (my guess is that so they can easily move threads between E & P cores).
It's pretty surprising that multiple CPU vendors have run into issues like this (some more than once, fucking Samsung), when it's pretty much the first thing that anyone on the toolchain side of thing asks when they hear about heterogenous cores on a CPU.
Since 1990 and until now, the Intel-AMD ISA and the hardware architecture of the Intel-AMD CPUs have accumulated a large number of very undesirable features, which increase the cost of hardware and reduce the security of computers, for which the only justification was the fact that Microsoft refused to update Windows so they expected that the hardware vendors must do whatever it takes to keep their CPUs compatible with the legacy operating systems.
These workarounds have moved inside the CPUs many functions that properly belong to the operating system, like remote management and voltage-frequency control.
Heterogeneous compute aware kernel scheduler seems like the most natural approach to me.
You could imagine the kernel being clever and handling the first SIGILL in hardware and then marking the process as big-core-only, but you can't handle this generally since you can have new instructions get encoded as nop (I believe the AArch64 PAC instructions do that?). You could also have an ifunc resolver intentionally call the instruction and expect and handle the signal.
The solution is to make your hardware actually work, and not claim to support instructions that do not actually exist on all of your cores.
After that happens, the thread can be marked as requiring AVX-512 and then it can be scheduled only on P-cores.
Moreover, one can define a new flag for the ELF header of executable files, specifying the AVX-512 requirement, which will be set by the compiler, depending on the compilation options, and then the OS knows from the beginning how to schedule a process.
Of course, the best solution is still to implement a uniform ISA. If a lower performance is accepted, the execution of the AVX-512 instructions can be implemented at a negligible additional cost over the previous x86_64 ISA.
What has scared the E-cores designers into not implementing AVX-512 was not the execution part, but the instruction decoding part. The instruction decoder for x86_64 is already horrendously complicated when many instructions must be decoded concurrently in a single clock cycle and adding the need to handle the completely distinct encoding of the AVX-512 makes it even more complicated.
Nonetheless, the competition from AMD has eventually forced Intel to give up in their attempt of reducing production costs by saving die area with a simpler instruction decoder in the E-cores, so the E-cores of Nova Lake will have the same ISA as its P-cores.
You're missing the point: the processes executing AVX-512 do not have any particularly strong requirement for AVX-512 instructions, they're just optimistically using them to improve performance because CPUID claimed they were supported. It would be pathologically bad behavior to control scheduling behavior with this, because in practice it would result in things like glibc deciding to use AVX-512 for string functions resulting in every program that uses memcpy (i.e. every program) not being allowed to be scheduled on the little cores.