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

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Date: Sat, 11 May 2024 01:24:12 +0000
From: "Li, Xin3" <xin3.li@...el.com>
To: "Gao, Chao" <chao.gao@...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"
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	"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()

> >+/*
> >+ * 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?

> Specifically,