RE: [PATCH v1 11/23] KVM: x86: Add kvm_is_fred_enabled()

Li, Xin3
2023-11-14T18:57:23Z

> >> 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:
>
> 1. the CPU enters 64-bit long mode and sets CR4.FRED
> 2. 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.