linux-kernel - Re: [PATCH v2 11/25] KVM: x86: Add kvm_is_fred

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Date: Sat, 11 May 2024 09:53:47 +0800
From: Chao Gao <chao.gao@...el.com>
To: "Li, Xin3" <xin3.li@...el.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "linux-kselftest@...r.kernel.org"
	<linux-kselftest@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "corbet@....net"
	<corbet@....net>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "x86@...nel.org"
	<x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>, "shuah@...nel.org"
	<shuah@...nel.org>, "vkuznets@...hat.com" <vkuznets@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>, "Shankar, Ravi V"
	<ravi.v.shankar@...el.com>, "xin@...or.com" <xin@...or.com>
Subject: Re: [PATCH v2 11/25] KVM: x86: Add kvm_is_fred_enabled()

On Sat, May 11, 2024 at 09:24:12AM +0800, Li, Xin3 wrote:
>> >+/*
>> >+ * It's enough to check just CR4.FRED (X86_CR4_FRED) to tell if
>> >+ * a vCPU is running with FRED enabled, because:
>> >+ * 1) CR4.FRED can be set to 1 only _after_ IA32_EFER.LMA = 1.
>> >+ * 2) To leave IA-32e mode, CR4.FRED must be cleared first.
>> >+ *
>> >+ * More details at FRED Spec 6.0 Section 4.2 Enabling in CR4.
>> >+ */
>> 
>> I think we can give more context here, e.g.,
>> 
>> Although FRED architecture applies to 64-bit mode only, there is no need to check if
>> the CPU is in 64-bit mode (i.e., IA32_EFER.LMA and CS.L) to tell if FRED is enabled
>> because CR4.FRED=1 implies the CPU is in 64-bit mode.
>
>What is "more context" here?

e.g.,
why IA32_EFER.LMA and CPU mode are related to FRED here?

"it's enough to " implies something else is not necessary. what is it?

I don't think the original comment make them super clear.

>
>> Specifically,
>
>