linux-kernel - Re: [PATCH v3 24/27] KVM: nVMX: Add a prerequisite to existence of VMCS fields

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Message-ID: <5b63fb37-bb1a-45f9-a6f9-58f6bf5b869e@zytor.com>
Date: Tue, 25 Feb 2025 08:37:16 -0800
From: Xin Li <xin@...or.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, pbonzini@...hat.com, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        luto@...nel.org, peterz@...radead.org, andrew.cooper3@...rix.com
Subject: Re: [PATCH v3 24/27] KVM: nVMX: Add a prerequisite to existence of
 VMCS fields

On 2/25/2025 8:22 AM, Sean Christopherson wrote:
> On Mon, Sep 30, 2024, Xin Li (Intel) wrote:
>> Add a prerequisite to existence of VMCS fields as some of them exist
>> only on processors that support certain CPU features.
>>
>> This is required to fix KVM unit test VMX_VMCS_ENUM.MAX_INDEX.
> 
> If making the KVM-Unit-Test pass is the driving force for this code, then NAK.
> We looked at this in detail a few years back, and came to the conclusion that
> trying to precisely track which fields are/aren't supported would likely do more
> harm than good.

I have to agree, it's no fun to track a VMCS field is added by which 
feature(s), and worst part is that one VMCS field could depend on 2+ 
totally irrelevant features, e.g., the secondary VM exit controls field 
exits on CPU that supports:

1) FRED
2) Prematurely busy shadow stack

Thanks for making the ground rule clear.

BTW, why don't we just remove this VMX_VMCS_ENUM.MAX_INDEX test?

     Xin


> 
> https://lore.kernel.org/all/1629192673-9911-4-git-send-email-robert.hu@linux.intel.com
>