Signal the AVIC doorbell iff the vCPU is running in the guest.  If the vCPU
is not IN_GUEST_MODE, it's guaranteed to pick up any pending IRQs on the
next VMRUN, which unconditionally processes the vIRR.

Add comments to document the logic.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/avic.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 208c5c71e827..cbf02e7e20d0 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c

@@ -674,7 +674,12 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
 	kvm_lapic_set_irr(vec, vcpu->arch.apic);
 	smp_mb__after_atomic();
 
-	if (avic_vcpu_is_running(vcpu)) {
+	/*
+	 * Signal the doorbell to tell hardware to inject the IRQ if the vCPU
+	 * is in the guest.  If the vCPU is not in the guest, hardware will
+	 * automatically process AVIC interrupts at VMRUN.
+	 */
+	if (vcpu->mode == IN_GUEST_MODE) {
 		int cpu = READ_ONCE(vcpu->cpu);
 
 		/*

@@ -687,8 +692,13 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
 		if (cpu != get_cpu())
 			wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
 		put_cpu();
-	} else
+	} else {
+		/*
+		 * Wake the vCPU if it was blocking.  KVM will then detect the
+		 * pending IRQ when checking if the vCPU has a wake event.
+		 */
 		kvm_vcpu_wake_up(vcpu);
+	}
 
 	return 0;
 }