Re: [PATCH 1/2] KVM: SVM: Initialize AVIC VMCB fields if AVIC is enabled with in-kernel APIC

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

On Tue, Feb 03, 2026 at 11:07:09AM -0800, Sean Christopherson wrote:
> Initialize all per-vCPU AVIC control fields in the VMCB if AVIC is enabled
> in KVM and the VM has an in-kernel local APIC, i.e. if it's _possible_ the
> vCPU could activate AVIC at any point in its lifecycle.  Configuring the
> VMCB if and only if AVIC is active "works" purely because of optimizations
> in kvm_create_lapic() to speculatively set apicv_active if AVIC is enabled
> *and* to defer updates until the first KVM_RUN.  In quotes because KVM

I think it will be good to clarify that two issues are being addressed 
here (it wasn't clear to me to begin with):
- One, described above, is about calling into avic_init_vmcb() 
  regardless of the vCPU APICv status.
- Two, described below is about using the vCPU APICv status for init and 
  not consulting the VM-level APICv inhibit status.

> likely won't do the right thing if kvm_apicv_activated() is false, i.e. if
> a vCPU is created while APICv is inhibited at the VM level for whatever
> reason.  E.g. if the inhibit is *removed* before KVM_REQ_APICV_UPDATE is
> handled in KVM_RUN, then __kvm_vcpu_update_apicv() will elide calls to
> vendor code due to seeing "apicv_active == activate".
> 
> Cleaning up the initialization code will also allow fixing a bug where KVM
> incorrectly leaves CR8 interception enabled when AVIC is activated without
> creating a mess with respect to whether AVIC is activated or not.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Any reason not to add a Fixes: tag? It looks like the below commits are 
to blame, but those are really old so I understand if you don't think 
this is useful:
Fixes: 67034bb9dd5e ("KVM: SVM: Add irqchip_split() checks before enabling AVIC")
Fixes: 6c3e4422dd20 ("svm: Add support for dynamic APICv")

Other than that:
Reviewed-by: Naveen N Rao (AMD) <naveen@kernel.org>

> ---
>  arch/x86/kvm/svm/avic.c | 2 +-
>  arch/x86/kvm/svm/svm.c  | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index f92214b1a938..44e07c27b190 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -368,7 +368,7 @@ void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb)
>  	vmcb->control.avic_physical_id = __sme_set(__pa(kvm_svm->avic_physical_id_table));
>  	vmcb->control.avic_vapic_bar = APIC_DEFAULT_PHYS_BASE;
>  
> -	if (kvm_apicv_activated(svm->vcpu.kvm))
> +	if (kvm_vcpu_apicv_active(&svm->vcpu))
>  		avic_activate_vmcb(svm);
>  	else
>  		avic_deactivate_vmcb(svm);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 5f0136dbdde6..e8313fdc5465 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1189,7 +1189,7 @@ static void init_vmcb(struct kvm_vcpu *vcpu, bool init_event)
>  	if (guest_cpu_cap_has(vcpu, X86_FEATURE_ERAPS))
>  		svm->vmcb->control.erap_ctl |= ERAP_CONTROL_ALLOW_LARGER_RAP;
>  
> -	if (kvm_vcpu_apicv_active(vcpu))
> +	if (enable_apicv && irqchip_in_kernel(vcpu->kvm))
>  		avic_init_vmcb(svm, vmcb);

Doesn't have to be done as part of this series, but I'm wondering if it 
makes sense to turn this into a helper to clarify the intent and to make 
it more obvious:

---
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e441f270f354..4e0ec4bf0db6 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2289,6 +2289,7 @@ gpa_t kvm_mmu_gva_to_gpa_write(struct kvm_vcpu *vcpu, gva_t gva,
 gpa_t kvm_mmu_gva_to_gpa_system(struct kvm_vcpu *vcpu, gva_t gva,
                                struct x86_exception *exception);

+bool kvm_apicv_possible(struct kvm *kvm);
 bool kvm_apicv_activated(struct kvm *kvm);
 bool kvm_vcpu_apicv_activated(struct kvm_vcpu *vcpu);
 void __kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 13a4a8949aba..f7b1271cea88 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -285,7 +285,7 @@ int avic_alloc_physical_id_table(struct kvm *kvm)
 {
        struct kvm_svm *kvm_svm = to_kvm_svm(kvm);

-       if (!irqchip_in_kernel(kvm) || !enable_apicv)
+       if (!kvm_apicv_possible(kvm))
                return 0;

        if (kvm_svm->avic_physical_id_table)
@@ -839,7 +839,7 @@ int avic_init_vcpu(struct vcpu_svm *svm)
        INIT_LIST_HEAD(&svm->ir_list);
        raw_spin_lock_init(&svm->ir_list_lock);

-       if (!enable_apicv || !irqchip_in_kernel(vcpu->kvm))
+       if (!kvm_apicv_possible(vcpu->kvm))
                return 0;

        ret = avic_init_backing_page(vcpu);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4115fe583052..b964d834512e 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1188,7 +1188,7 @@ static void init_vmcb(struct kvm_vcpu *vcpu, bool init_event)
        if (guest_cpu_cap_has(vcpu, X86_FEATURE_ERAPS))
                svm->vmcb->control.erap_ctl |= ERAP_CONTROL_ALLOW_LARGER_RAP;

-       if (enable_apicv && irqchip_in_kernel(vcpu->kvm))
+       if (kvm_apicv_possible(vcpu->kvm))
                avic_init_vmcb(svm, vmcb);

        if (vnmi)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8acfdfc583a1..86f99c5b831a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10270,6 +10270,12 @@ static void kvm_pv_kick_cpu_op(struct kvm *kvm, int apicid)
        kvm_irq_delivery_to_apic(kvm, NULL, &lapic_irq);
 }

+bool kvm_apicv_possible(struct kvm *kvm)
+{
+       return enable_apicv && irqchip_in_kernel(kvm);
+}
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_apicv_possible);
+
 bool kvm_apicv_activated(struct kvm *kvm)
 {
	return (READ_ONCE(kvm->arch.apicv_inhibit_reasons) == 0);


- Naveen