[PATCH v2 1/3] KVM: SVM: Add paranoid helper for checking if vCPU is AVIC-addressable

[Sean Christopherson <seanjc@google.com>]

Add a helper to check if a vCPU is addressable by AVIC hardware, i.e. has
an APIC ID that fits in the physical ID table, and use the more paranoid
helper when determining if a vCPU is compatible with AVIC when initializing
the vCPU.  KVM is supposed to reject vCPU creation if the vCPU's ID is
greater than or equal to max_vcpu_ids, i.e. simply checking the
architectural maximum *should* suffice.  But piecing together why this is
safe is unnecessarily difficult, and there is no meaningful downside to
being extra cautious.

Suggested-by: Naveen N Rao (AMD) <naveen@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/avic.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 3b037e385523..0e4b5eb6ac82 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -395,6 +395,11 @@ static phys_addr_t avic_get_backing_page_address(struct vcpu_svm *svm)
 	return __sme_set(__pa(svm->vcpu.arch.apic->regs));
 }
 
+static bool avic_is_addressable_vcpu(struct kvm_vcpu *vcpu)
+{
+	return vcpu->vcpu_id <= __avic_get_max_physical_id(vcpu->kvm, NULL);
+}
+
 void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb)
 {
 	struct kvm_svm *kvm_svm = to_kvm_svm(svm->vcpu.kvm);
@@ -412,7 +417,6 @@ 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;
@@ -425,7 +429,7 @@ 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 (id > max_id) {
+	if (!avic_is_addressable_vcpu(vcpu)) {
 		kvm_set_apicv_inhibit(vcpu->kvm, APICV_INHIBIT_REASON_PHYSICAL_ID_TOO_BIG);
 		vcpu->arch.apic->apicv_active = false;
 		return 0;
-- 
2.55.0.1082.g2b9226bbc0-goog