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

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Message-ID: <Z74au5wLvtFuJuS8@google.com>
Date: Tue, 25 Feb 2025 11:32:11 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xin Li <xin@...or.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 Tue, Feb 25, 2025, Xin Li wrote:
> 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?

Because it's still a valid test, albeit with caveats.  KVM's (undocumented?) erratum
is that vmcs12 fields that are supported by KVM are always readable, but that's
mostly an orthogonal issuue to VMX_VMCS_ENUM.MAX_INDEX.  I.e. KVM can and does
report a correct VMX_VMCS_ENUM.MAX_INDEX based on which VMCS fields KVM emulates.

The big caveat is that VMX_VMCS_ENUM.MAX_INDEX will be wrong if a VM is migrated
to a newer KVM and/or to a host with a superset of functionality.  With those
caveats in mind, it's still nice to sanity check that KVM isn't advertising complete
garbage.