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

Chao Gao
2023-11-14T08:17:02Z

On Tue, Nov 14, 2023 at 12:42:13PM +0800, Li, Xin3 wrote:
>> >+ return cpu_feature_enabled(X86_FEATURE_FRED) &&
>> >+ kvm_is_cr4_bit_set(vcpu, X86_CR4_FRED);
>>
>> 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.

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.

>
>If you are talking about save/restore a corrupted vCPU state, a following
>VM entry should fail anyway.