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

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Message-ID: <76e007d7fc7af0629279f2563f8d0c48274bc774.camel@redhat.com>
Date:   Sun, 10 Jul 2022 19:08:17 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     "Shukla, Santosh" <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 Fri, 2022-06-17 at 20:29 +0530, Shukla, Santosh wrote:
> 
> On 6/7/2022 6:42 PM, Maxim Levitsky wrote:
> > 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?
> > > 
> Yes.
> 
> > > 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?
> > > 
> No.
> 
> For both case - HW takes the V_NMI event at the boundary of VMRUN instruction.

How that is possible? if vGIF is disabled in L1, then L1 can't execute STGI/CLGI - 
that means that the CPU can't update the V_NMI, as it never sees the STGI/CLGI
beeing executed.

Best regards,
	Maxim Levitsky

> 
> > > 
> > > 
> > > > +
> > > >         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?
> > 
> If V_NMI_MASK set, that means V_NMI is pending, if so then inject another V_NMI in next VMRUN.
> 
> Thanks,
> Santosh
> 
> > 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