FRED is enabled when CR4.FRED = IA32_EFER.LMA = 1. Any reason to omit the check about long mode?
It won' t allow CR4.FRED to be set if not in long mode, I don't expect it at runtime. Or you have one?
I was thinking about a very contrived case:
- the CPU enters 64-bit long mode and sets CR4.FRED
- the CPU switches out of 64-bit long mode
and SDM vol3 chapter 2.5 CONTROL REGISTERS says:
A 64-bit capable processor will retain the upper 32 bits of each control register when transitioning out of IA-32e mode.
so, to me, it is possible that CR4.FRED is 1 while IA32_EFER.LMA is 0. and in this case, FRED should be considered disabled.
You're correct, this is a solid case.
It's not one-way, but I forgot the other way around.
Anyway, I think we should align with FRED SPEC. If we deliberately omit the check about long mode, please add a comment to explain why it is ok to do that.
Yeah, I will add it.