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

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Message-ID: <ZNzgWwUNy/2J3SV2@google.com>
Date:   Wed, 16 Aug 2023 07:42:35 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Guang Zeng <guang.zeng@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yuan Yao <yuan.yao@...el.com>
Subject: Re: [PATCH v3 07/15] KVM: nVMX: Use KVM-governed feature framework to
 track "nested VMX enabled"

On Wed, Aug 16, 2023, Kai Huang wrote:
> 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.

Yeah, 100% agree after re-reading this patch without context.  I had to go search
the code to remember where "nested" is checked.  :-)

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

Yep, I'll add a blurb to point out the dependency in vmx_set_cpu_caps().

Thanks!