[PATCH 3/6] KVM: SVM: fix AVIC race of host->guest IPI delivery vs AVIC inhibition

[Maxim Levitsky]

If svm_deliver_avic_intr is called just after the target vcpu's AVIC got
inhibited, it might read a stale value of vcpu->arch.apicv_active
which can lead to the target vCPU not noticing the interrupt.

Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
---
 arch/x86/kvm/svm/avic.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 859ad2dc50f1..8c1b934bfa9b 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -691,6 +691,15 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
 	 * automatically process AVIC interrupts at VMRUN.
 	 */
 	if (vcpu->mode == IN_GUEST_MODE) {
+
+		/*
+		 * At this point we had read the vcpu->arch.apicv_active == true
+		 * and the vcpu->mode == IN_GUEST_MODE.
+		 * Since we have a memory barrier after setting IN_GUEST_MODE,
+		 * it ensures that AVIC inhibition is complete and thus
+		 * the target is really running with AVIC enabled.
+		 */
+
 		int cpu = READ_ONCE(vcpu->cpu);
 
 		/*
@@ -706,10 +715,11 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
 		put_cpu();
 	} else {
 		/*
-		 * Wake the vCPU if it was blocking.  KVM will then detect the
-		 * pending IRQ when checking if the vCPU has a wake event.
+		 * Kick the target vCPU otherwise, to make sure
+		 * it processes the interrupt even if its AVIC is inhibited.
 		 */
-		kvm_vcpu_wake_up(vcpu);
+		kvm_make_request(KVM_REQ_EVENT, vcpu);
+		kvm_vcpu_kick(vcpu);
 	}
 
 	return 0;
-- 
2.26.3