KVM: SVM: Signal AVIC doorbell iff vCPU is in guest mode

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>
Message-Id: <20211208015236.1616697-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/avic.c

@@ -672,9 +672,22 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
 		return -1;
 
 	kvm_lapic_set_irr(vec, vcpu->arch.apic);
+
+	/*
+	 * Pairs with the smp_mb_*() after setting vcpu->guest_mode in
+	 * vcpu_enter_guest() to ensure the write to the vIRR is ordered before
+	 * the read of guest_mode, which guarantees that either VMRUN will see
+	 * and process the new vIRR entry, or that the below code will signal
+	 * the doorbell if the vCPU is already running in the guest.
+	 */
 	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);
 
 		/*
@@ -688,8 +701,13 @@ 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;
 }