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

[Naveen N Rao <naveen@kernel.org>]

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@google.com>
> >> 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
> > 
> > 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()


- Naveen