linux-kernel - Re: [PATCH v1 11/23] KVM: x86: Add kvm_is_fred

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Message-ID: <ZVMs6vZyoqd2cUiV@chao-email>
Date:   Tue, 14 Nov 2023 16:16:42 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     "Li, Xin3" <xin3.li@...el.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH v1 11/23] KVM: x86: Add kvm_is_fred_enabled()

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.