linux-kernel - Re: [PATCH] KVM: x86: Disallow setting CPUID and/or feature MSRs if L2 is active

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Message-ID: <z3i7gmjq6e4hsokfemknvpdeubnifivvl4xxf4pvrlqdtv4mkq@bvhftx7fsdbk>
Date: Thu, 8 Jan 2026 07:59:26 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Kevin Cheng <chengkev@...gle.com>
Subject: Re: [PATCH] KVM: x86: Disallow setting CPUID and/or feature MSRs if
 L2 is active

On Wed, Jan 07, 2026 at 12:26:35PM -0800, Sean Christopherson wrote:
> On Fri, Jan 02, 2026, Yosry Ahmed wrote:
> > On Tue, Dec 30, 2025 at 12:56:41PM -0800, Sean Christopherson wrote:
> 
> > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > > index fdab0ad49098..9084e0dfa15c 100644
> > > --- a/arch/x86/kvm/x86.h
> > > +++ b/arch/x86/kvm/x86.h
> > > @@ -172,9 +172,9 @@ static inline void kvm_nested_vmexit_handle_ibrs(struct kvm_vcpu *vcpu)
> > >  		indirect_branch_prediction_barrier();
> > >  }
> > >  
> > > -static inline bool kvm_vcpu_has_run(struct kvm_vcpu *vcpu)
> > > +static inline bool kvm_can_set_cpuid_and_feature_msrs(struct kvm_vcpu *vcpu)
> > >  {
> > > -	return vcpu->arch.last_vmentry_cpu != -1;
> > > +	return vcpu->arch.last_vmentry_cpu == -1 && !is_guest_mode(vcpu);
> > >  }
> > 
> > To make this self-contained (e.g. for readers not coming from
> > kvm_set_cpuid()), should we add a comment here about is_guest_mode()
> > only possibly being true with last_vmentry_cpu == -1 if userspace does
> > the set CPUID, set nested state, set CPUID again dance?
> 
> Ya.  If this looks good, I'll add it when applying.
> 
> /*
>  * Disallow modifying CPUID and feature MSRs, which affect the core virtual CPU
>  * model exposed to the guest and virtualized by KVM, if the vCPU has already
>  * run or is in guest mode (L2).  In both cases, KVM has already consumed the
>  * current virtual CPU model, and doesn't support "unwinding" to react to the
>  * new model.
>  *
>  * Note, the only way is_guest_mode() can be true with 'last_vmentry_cpu == -1'
>  * is if userspace sets CPUID and feature MSRs (to enable VMX/SVM), then sets
>  * nested state, and then attempts to set CPUID and/or feature MSRs *again*.
>  */
> static inline bool kvm_can_set_cpuid_and_feature_msrs(struct kvm_vcpu *vcpu)
> {
> 	return vcpu->arch.last_vmentry_cpu == -1 && !is_guest_mode(vcpu);
> }

Looks good to me.