Re: [PATCH 2/2] KVM: SVM: Set/clear CR8 write interception when AVIC is (de)activated

[Naveen N Rao]

On Tue, Mar 10, 2026 at 11:18:16PM +0530, Naveen N Rao wrote:
> On Tue, Mar 10, 2026 at 12:36:09PM -0500, Tom Lendacky wrote:
> > On 3/10/26 12:17, Sean Christopherson wrote:
> > > On Tue, Mar 10, 2026, Srikanth Aithal wrote:
> > >>
> > >> Hello Sean,
> > >>
> > >> From next-20260304 onwards [1], including recent next kernel next-20260309,
> > >> booting an SEV-ES guest on AMD EPYC Turin and AMD EPYC Genoa has been
> > >> failing. However, on EPYC Milan, the SEV-ES guest boots fine.
> > > 
> > > ...
> > > 
> > >> Bisecting shows that this commit is the first bad one. When I revert it, I
> > >> am able to boot the SEV-ES guest successfully on both Turin and Genoa
> > >> platforms:
> > >>
> > >> e992bf67bcbab07a7f59963b2c4ed32ef65c8431 is the first bad commit
> > >> commit e992bf67bcbab07a7f59963b2c4ed32ef65c8431
> > >> Author: Sean Christopherson <seanjc@xxxxxxxxxx>
> > >> Date:   Tue Feb 3 11:07:10 2026 -0800
> > > 
> > > Gah, I hate how KVM manages intercepts for SEV-ES+.  Though to a large extent I
> > > blame the architecture for not simply making CR{0,4,8} intercept trap-like.
> > > Side topic, is the host actually allowed to trap CR3 writes?  That seems like a
> > > huge gaping security flaw, especially for SNP+.
> > > 
> > > Anyways, this should fix the immediate problem.
> > > 
> > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > > index 33172f0e986b..b6072872b785 100644
> > > --- a/arch/x86/kvm/svm/avic.c
> > > +++ b/arch/x86/kvm/svm/avic.c
> > > @@ -237,7 +237,8 @@ static void avic_deactivate_vmcb(struct vcpu_svm *svm)
> > >         vmcb->control.int_ctl &= ~(AVIC_ENABLE_MASK | X2APIC_MODE_MASK);
> > >         vmcb->control.avic_physical_id &= ~AVIC_PHYSICAL_MAX_INDEX_MASK;
> > >  
> > > -       svm_set_intercept(svm, INTERCEPT_CR8_WRITE);
> > > +       if (!sev_es_guest(svm->vcpu.kvm))
> > > +               svm_set_intercept(svm, INTERCEPT_CR8_WRITE);
> > >  
> > >         /*
> > >          * If running nested and the guest uses its own MSR bitmap, there

I arrived at the same fix and it works for me, so FWIW:
Acked-by: Naveen N Rao (AMD) <naveen@xxxxxxxxxx>

> > > 
> > > Argh!  The more I look at this code, the more frustrated I get.  
> > > The unconditional
> > > setting of TRAP_CR8_WRITE for SEV-ES+ is flawed.  When AVIC is enabled, KVM doesn't
> > 
> > AVIC is disabled for SEV guests (see __sev_guest_init() and the
> > kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_SEV) call at the end of
> > the function).
> 
> AVIC gets inhibited globally, but continues to be enabled on 
> vcpu_create() opportunistically -- see kvm_create_lapic(). It only gets 
> disabled later during vcpu setup via 
> vcpu_reset()->svm_vcpu_reset()->init_vmcb()->avic_init_vmcb()

... which explains why this issue is showing up.

But reading your response again, I guess you were pointing out that the 
intercepts are not a problem for SEV-ES guests since AVIC is inhibited, 
which totally makes sense.

Thanks,
Naveen