On: Google developing own CPUs for Chromebook laptops

[ithkuil]

If the extra instructions also operate in extra state (e.g. extra registers) a kernel needs to know about their existence so it can correctly save and restore state on context switches

[my123 → ithkuil]

Not necessarily/really, the custom extensions still need to be enabled by the kernel before they can be used.

As such, it isn’t actually an issue.

[ndesaulniers → ithkuil]

You're confusing MSRs (which don't have to be saved/restored on context switch) with GPRs (which do).

[ithkuil → ndesaulniers]

Well if there is a truly undocumented extension, how do I know it doesn't come with it's own registers (e.g. like a floating point unit does)

[saagarjha → ndesaulniers]

Apple reserves certain MSRs that apply on a per-process basis, and thus must be applied on context switch.