linux-kernel - Re: [PATCH v3 07/15] KVM: nVMX: Use KVM-governed feature framework to track "nested VMX enabled"

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Date:   Wed, 16 Aug 2023 02:45:38 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yao, Yuan" <yuan.yao@...el.com>
Subject: Re: [PATCH v3 07/15] KVM: nVMX: Use KVM-governed feature framework to
 track "nested VMX enabled"

On Tue, 2023-08-15 at 13:36 -0700, Sean Christopherson wrote:
> Track "VMX exposed to L1" via a governed feature flag instead of using a
> dedicated helper to provide the same functionality.  The main goal is to
> drive convergence between VMX and SVM with respect to querying features
> that are controllable via module param (SVM likes to cache nested
> features), avoiding the guest CPUID lookups at runtime is just a bonus
> and unlikely to provide any meaningful performance benefits.
> 
> No functional change intended.
> 
> 
[...]

>  
> -/*
> - * nested_vmx_allowed() checks whether a guest should be allowed to use VMX
> - * instructions and MSRs (i.e., nested VMX). Nested VMX is disabled for
> - * all guests if the "nested" module option is off, and can also be disabled
> - * for a single guest by disabling its VMX cpuid bit.
> - */
> -bool nested_vmx_allowed(struct kvm_vcpu *vcpu)
> -{
> -	return nested && guest_cpuid_has(vcpu, X86_FEATURE_VMX);
> -}
> -
> 

[...]

> @@ -7750,13 +7739,15 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	    guest_cpuid_has(vcpu, X86_FEATURE_XSAVE))
>  		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_XSAVES);
>  
> +	kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_VMX);
> +
> 

Nit:

nested_vmx_allowed() also checks 'nested' global variable.  However
kvm_governed_feature_check_and_set() is called unconditionally.  Although IIUC
it should never actually set the VMX governed bit when 'nested=0', it's not that
obvious in _this_ patch.

Should we explicitly call this out in the changelog so git blamers can
understand this more easily in the future?