[PATCH] KVM: SVM: Check vCPU ID against max x2AVIC ID if and only if x2AVIC is enabled

Sean Christopherson
2026-01-12T23:28:08Z

When allocating the AVIC backing page, only check one of the max AVIC vs.
x2AVIC ID based on whether or not x2AVIC is enabled. Doing so fixes a bug
where KVM incorrectly inhibits AVIC if x2AVIC is _disabled_ and any vCPU
with a non-zero APIC ID is created, as x2avic_max_physical_id is left '0'
when x2AVIC is disabled.

Fixes: 940fc47cfb0d ("KVM: SVM: Add AVIC support for 4k vCPUs in x2AVIC mode")
Cc: stable@vger.kernel.org
Cc: Naveen N Rao (AMD) <naveen@kernel.org>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/avic.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 6b77b2033208..0f6c8596719b 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -376,6 +376,7 @@ void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb)

static int avic_init_backing_page(struct kvm_vcpu *vcpu)
{
+ u32 max_id = x2avic_enabled ? x2avic_max_physical_id : AVIC_MAX_PHYSICAL_ID;
struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
struct vcpu_svm *svm = to_svm(vcpu);
u32 id = vcpu->vcpu_id;
@@ -388,8 +389,7 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
* avic_vcpu_load() expects to be called if and only if the vCPU has
* fully initialized AVIC.
*/
- if ((!x2avic_enabled && id > AVIC_MAX_PHYSICAL_ID) ||
- (id > x2avic_max_physical_id)) {
+ if (id > max_id) {
kvm_set_apicv_inhibit(vcpu->kvm, APICV_INHIBIT_REASON_PHYSICAL_ID_TOO_BIG);
vcpu->arch.apic->apicv_active = false;
return 0;

base-commit: 9448598b22c50c8a5bb77a9103e2d49f134c9578
--
2.52.0.457.g6b5491de43-goog