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

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Message-ID: <SA1PR11MB6734EBD796D139957BFF2457A8B2A@SA1PR11MB6734.namprd11.prod.outlook.com>
Date:   Tue, 14 Nov 2023 18:57:13 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "Gao, Chao" <chao.gao@...el.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
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        "corbet@....net" <corbet@....net>,
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        Dexuan Cui <decui@...rosoft.com>,
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        "peterz@...radead.org" <peterz@...radead.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [PATCH v1 11/23] KVM: x86: Add kvm_is_fred_enabled()


> >> 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.