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

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>
Reviewed-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://patch.msgid.link/20260112232805.1512361-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 6b77b2033208f15312b1ffad1dac53d33e3037bb..0f6c8596719b833afac5d48a21a2eef38e44a568 100644 (file)
--- 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;