[klauspost]

x86 Darwin had a bug where AVX512 K (opmask) registers would not be saved and restored on context switches when YMM registers had all 0 values. The kernel assumed that AVX512 wasn't used. They fixed it in Monterey 12.2, but still hasn't backported. Heisenbug for sure.

Ref: https://github.com/golang/go/issues/49233#issuecomment-96373...

[KMag]

It's unfortunate that more architectures aren't implemented with something like DEC Alpha's PALCode, where the CPU's firmware is essentially a single-tenant hypervisor. The OS kernel proper uses upcalls to the firmware for manipulating page tables, etc. If abstracted properly, the OS would allocate the storage space, but ask the firmware to serialize and deserialize processor state on context switches.

In retrospect, the Alpha went a bit too far with the permissiveness of its memory model, and it turned out they really did need single-byte load and store instructions. However, it was really an elegant high-level design, and the implementation team was top-notch (same folks worked on StrongArm, AMD Athlon, P.A. Semi's PWRficient, and Apple silicon).