KVM: lapic: Explicitly cancel the hv timer if it's pre-expired

Explicitly call cancel_hv_timer() instead of returning %false to coerce
restart_apic_timer() into canceling it by way of start_sw_timer().

Functionally, the existing code is correct in the sense that it doesn't
doing anything visibily wrong, e.g. generate spurious interrupts or miss
an interrupt.  But it's extremely confusing and inefficient, e.g. there
are multiple extraneous calls to apic_timer_expired() that effectively
get dropped due to @timer_pending being %true.

Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -1693,17 +1693,25 @@ static bool start_hv_timer(struct kvm_lapic *apic)
 	hrtimer_cancel(&ktimer->timer);
 
 	/*
-	 * Also recheck ktimer->pending, in case the sw timer triggered in
-	 * the window.  For periodic timer, leave the hv timer running for
-	 * simplicity, and the deadline will be recomputed on the next vmexit.
+	 * To simplify handling the periodic timer, leave the hv timer running
+	 * even if the deadline timer has expired, i.e. rely on the resulting
+	 * VM-Exit to recompute the periodic timer's target expiration.
 	 */
-	if (!apic_lvtt_period(apic) && (r || atomic_read(&ktimer->pending))) {
-		if (r)
+	if (!apic_lvtt_period(apic)) {
+		/*
+		 * Cancel the hv timer if the sw timer fired while the hv timer
+		 * was being programmed, or if the hv timer itself expired.
+		 */
+		if (atomic_read(&ktimer->pending)) {
+			cancel_hv_timer(apic);
+		} else if (r) {
 			apic_timer_expired(apic);
-		return false;
+			cancel_hv_timer(apic);
+		}
 	}
 
-	trace_kvm_hv_timer_state(apic->vcpu->vcpu_id, true);
+	trace_kvm_hv_timer_state(apic->vcpu->vcpu_id, ktimer->hv_timer_in_use);
+
 	return true;
 }