KVM: x86: Add comment on vcpu_enter_guest()'s return value

Giving proper names to the 0 and 1 was once suggested.  But since 0 is
returned to the userspace, giving it another name can introduce extra
confusion.  This patch just explains the meanings instead.
Signed-off-by: NTakuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -5865,6 +5865,11 @@ static void vcpu_scan_ioapic(struct kvm_vcpu *vcpu)
 	kvm_apic_update_tmr(vcpu, tmr);
 }
 
+/*
+ * Returns 1 to let __vcpu_run() continue the guest execution loop without
+ * exiting to the userspace.  Otherwise, the value will be returned to the
+ * userspace.
+ */
 static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 {
 	int r;