linux-kernel - Re: [PATCH 4/7] KVM: SVM: Report NMI not allowed when Guest busy handling VNMI

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Message-ID: <45e9ccafcdb48c7521b697b41e849dab98a7a76c.camel@redhat.com>
Date:   Tue, 07 Jun 2022 16:12:07 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Santosh Shukla <santosh.shukla@....com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] KVM: SVM: Report NMI not allowed when Guest busy
 handling VNMI

On Tue, 2022-06-07 at 16:10 +0300, Maxim Levitsky wrote:
> On Thu, 2022-06-02 at 19:56 +0530, Santosh Shukla wrote:
> > In the VNMI case, Report NMI is not allowed when the processor set the
> > V_NMI_MASK to 1 which means the Guest is busy handling VNMI.
> > 
> > Signed-off-by: Santosh Shukla <santosh.shukla@....com>
> > ---
> >  arch/x86/kvm/svm/svm.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index d67a54517d95..a405e414cae4 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -3483,6 +3483,9 @@ bool svm_nmi_blocked(struct kvm_vcpu *vcpu)
> >         struct vmcb *vmcb = svm->vmcb;
> >         bool ret;
> >  
> > +       if (is_vnmi_enabled(vmcb) && is_vnmi_mask_set(vmcb))
> > +               return true;
> 
> How does this interact with GIF? if the guest does clgi, will the
> CPU update the V_NMI_MASK on its own If vGIF is enabled?
> 
> What happens if vGIF is disabled and vNMI is enabled? KVM then intercepts
> the stgi/clgi, and it should then update the V_NMI_MASK?
> 
> 
> 
> 
> > +
> >         if (!gif_set(svm))
> >                 return true;
> >  
> > @@ -3618,6 +3621,9 @@ static void svm_enable_nmi_window(struct kvm_vcpu *vcpu)
> >  {
> >         struct vcpu_svm *svm = to_svm(vcpu);
> >  
> > +       if (is_vnmi_enabled(svm->vmcb) && is_vnmi_mask_set(svm->vmcb))
> > +               return;
> 
> This might have hidden assumption that we will only enable NMI window when vNMI is masked.

Also what if vNMI is already pending?

Best regards,
	Maxim Levitsky
> 
> 
> 
> > +
> >         if ((vcpu->arch.hflags & (HF_NMI_MASK | HF_IRET_MASK)) == HF_NMI_MASK)
> >                 return; /* IRET will cause a vm exit */
> >  
> 
> 
> Best regards,
>         Maxim Levitsky