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
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>
---
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);
if (vnmi)
--
2.53.0.rc2.204.g2597b5adb4-googOn 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
On Fri, Feb 06, 2026, Naveen N Rao wrote:
> 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.
Yeah, I was worried the changelog didn't capture the second one well, but I was
struggling to come up with wording. How about this as a penultimate paragraph?
Note! Use the vCPU's current APICv status when initializing the VMCB,
not the VM-level inhibit status. The state of the VMCB *must* be kept
consistent with the vCPU's APICv status at all times (KVM elides updates
that are supposed be nops). If the vCPU's APICv status isn't up-to-date
with the VM-level status, then there is guaranteed to be a pending
KVM_REQ_APICV_UPDATE, i.e. KVM will sync the vCPU with the VM before
entering the guest.
> > 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?
Purely that I couldn't pin down exactly what commit(s) to blame. Well, that's a
bit of a lie. If I'm being 100% truthful, I got as far as commit 67034bb9dd5e
and decided I didn't care enough to spend the effort to figure out whether or not
that commit was truly to blame :-)
> 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")
LGTM, I'll tack them on.
> Other than that:
> Reviewed-by: Naveen N Rao (AMD) <naveen@kernel.org>
Thanks! (Seriously, I really appreciate the in-depth reviews)
> > ---
> > 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:
Hmm, yeah, though my only hesitation is the name. For whatever reason, "possible"
makes me think "is APICv possible *right now*" (ignoring that I wrote exactly that
in the changelog).
What if we go with kvm_can_use_apicv()? That would align with vmx_can_use_ipiv()
and vmx_can_use_vtd_pi(), which are pretty much identical in concept.
On Tue, Feb 3, 2026 at 11:07 AM Sean Christopherson <seanjc@google.com> 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 > 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> Reviewed-by: Jim Mattson <jmattson@google.com>
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