That's actually an interesting idea, and yeah that should work.
You'd still have the problem that software will use the CPUID instruction to do runtime-detection of AVX-512 support. You'd need some mechanism to make CPUID report to lack AVX-512 support if the OS doesn't support catching SIGILL in the way you describe, and make CPUID report to support AVX-512 (even when run on an E-core) if the OS supports catching SIGILL and moving to a P-core. That sounds doable, but I have no idea how easy it is. You'd need to be able to configure AVX-512 reporting differently for virtual machine guests than for the host, and you'd need the host to be able to reconfigure AVX-512 support at runtime to support e.g kexec. There are probably tonnes of other considerations as well which I'm not thinking about.
Given the relatively limited benefit from going 512-bit wide compared to 256-bit, I guess I understand the decision, but you're right that it's not as black and white as I made it out to be.
You'd still have the problem that software will use the CPUID instruction to do runtime-detection of AVX-512 support. You'd need some mechanism to make CPUID report to lack AVX-512 support if the OS doesn't support catching SIGILL in the way you describe, and make CPUID report to support AVX-512 (even when run on an E-core) if the OS supports catching SIGILL and moving to a P-core. That sounds doable, but I have no idea how easy it is. You'd need to be able to configure AVX-512 reporting differently for virtual machine guests than for the host, and you'd need the host to be able to reconfigure AVX-512 support at runtime to support e.g kexec. There are probably tonnes of other considerations as well which I'm not thinking about.
Given the relatively limited benefit from going 512-bit wide compared to 256-bit, I guess I understand the decision, but you're right that it's not as black and white as I made it out to be.