KVM: SVM: extract avic_ring_doorbell

The check on the current CPU adds an extra level of indentation to
svm_deliver_avic_intr and conflates documentation on what happens
if the vCPU exits (of interest to svm_deliver_avic_intr) and migrates
(only of interest to avic_ring_doorbell, which calls get/put_cpu()).
Extract the wrmsr to a separate function and rewrite the
comment in svm_deliver_avic_intr().

Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: Maxim Levitsky

arch/x86/kvm/svm/avic.c

@@ -269,6 +269,22 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
+static void avic_ring_doorbell(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * Note, the vCPU could get migrated to a different pCPU at any point,
+	 * which could result in signalling the wrong/previous pCPU.  But if
+	 * that happens the vCPU is guaranteed to do a VMRUN (after being
+	 * migrated) and thus will process pending interrupts, i.e. a doorbell
+	 * is not needed (and the spurious one is harmless).
+	 */
+	int cpu = READ_ONCE(vcpu->cpu);
+
+	if (cpu != get_cpu())
+		wrmsrl(MSR_AMD64_SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
+	put_cpu();
+}
+
 static void avic_kick_target_vcpus(struct kvm *kvm, struct kvm_lapic *source,
 				   u32 icrl, u32 icrh)
 {
@@ -669,19 +685,12 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
 	 * automatically process AVIC interrupts at VMRUN.
 	 */
 	if (vcpu->mode == IN_GUEST_MODE) {
-		int cpu = READ_ONCE(vcpu->cpu);
-
 		/*
-		 * Note, the vCPU could get migrated to a different pCPU at any
-		 * point, which could result in signalling the wrong/previous
-		 * pCPU.  But if that happens the vCPU is guaranteed to do a
-		 * VMRUN (after being migrated) and thus will process pending
-		 * interrupts, i.e. a doorbell is not needed (and the spurious
-		 * one is harmless).
+		 * Signal the doorbell to tell hardware to inject the IRQ.  If
+		 * the vCPU exits the guest before the doorbell chimes, hardware
+		 * will automatically process AVIC interrupts at the next VMRUN.
 		 */
-		if (cpu != get_cpu())
-			wrmsrl(MSR_AMD64_SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
-		put_cpu();
+		avic_ring_doorbell(vcpu);
 	} else {
 		/*
 		 * Wake the vCPU if it was blocking.  KVM will then detect the