KVM: x86/xen: Use fast path for Xen timer delivery

Most of the time there's no need to kick the vCPU and deliver the timer
event through kvm_xen_inject_timer_irqs(). Use kvm_xen_set_evtchn_fast()
directly from the timer callback, and only fall back to the slow path if
delivering the timer would block, i.e. if kvm_xen_set_evtchn_fast()
returns -EWOULDBLOCK.  If delivery fails for any other reason, do nothing
and just let it fail silently, as that is what the slow path would end up
doing anyways.

This gives a significant improvement in timer latency testing (using
nanosleep() for various periods and then measuring the actual time
elapsed).

However, there was a reason[1] the fast path was dropped when this support
was first added. The current code holds vcpu->mutex for all operations on
the kvm->arch.timer_expires field, and the fast path introduces a
potential race condition. Avoid that race by ensuring the hrtimer is
(temporarily) cancelled before making changes in kvm_xen_start_timer(),
and also when reading the values out for KVM_XEN_VCPU_ATTR_TYPE_TIMER.

[1] https://lore.kernel.org/kvm/846caa99-2e42-4443-1070-84e49d2f11d2@redhat.com

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/f21ee3bd852761e7808240d4ecaec3013c649dc7.camel@infradead.org
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/xen.c

@@ -134,9 +134,23 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer)
 {
 	struct kvm_vcpu *vcpu = container_of(timer, struct kvm_vcpu,
 					     arch.xen.timer);
+	struct kvm_xen_evtchn e;
+	int rc;
+
 	if (atomic_read(&vcpu->arch.xen.timer_pending))
 		return HRTIMER_NORESTART;
 
+	e.vcpu_id = vcpu->vcpu_id;
+	e.vcpu_idx = vcpu->vcpu_idx;
+	e.port = vcpu->arch.xen.timer_virq;
+	e.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL;
+
+	rc = kvm_xen_set_evtchn_fast(&e, vcpu->kvm);
+	if (rc != -EWOULDBLOCK) {
+		vcpu->arch.xen.timer_expires = 0;
+		return HRTIMER_NORESTART;
+	}
+
 	atomic_inc(&vcpu->arch.xen.timer_pending);
 	kvm_make_request(KVM_REQ_UNBLOCK, vcpu);
 	kvm_vcpu_kick(vcpu);
@@ -146,6 +160,14 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer)
 
 static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs, s64 delta_ns)
 {
+	/*
+	 * Avoid races with the old timer firing. Checking timer_expires
+	 * to avoid calling hrtimer_cancel() will only have false positives
+	 * so is fine.
+	 */
+	if (vcpu->arch.xen.timer_expires)
+		hrtimer_cancel(&vcpu->arch.xen.timer);
+
 	atomic_set(&vcpu->arch.xen.timer_pending, 0);
 	vcpu->arch.xen.timer_expires = guest_abs;
 
@@ -1019,9 +1041,36 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		break;
 
 	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
+		/*
+		 * Ensure a consistent snapshot of state is captured, with a
+		 * timer either being pending, or the event channel delivered
+		 * to the corresponding bit in the shared_info. Not still
+		 * lurking in the timer_pending flag for deferred delivery.
+		 * Purely as an optimisation, if the timer_expires field is
+		 * zero, that means the timer isn't active (or even in the
+		 * timer_pending flag) and there is no need to cancel it.
+		 */
+		if (vcpu->arch.xen.timer_expires) {
+			hrtimer_cancel(&vcpu->arch.xen.timer);
+			kvm_xen_inject_timer_irqs(vcpu);
+		}
+
 		data->u.timer.port = vcpu->arch.xen.timer_virq;
 		data->u.timer.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL;
 		data->u.timer.expires_ns = vcpu->arch.xen.timer_expires;
+
+		/*
+		 * The hrtimer may trigger and raise the IRQ immediately,
+		 * while the returned state causes it to be set up and
+		 * raised again on the destination system after migration.
+		 * That's fine, as the guest won't even have had a chance
+		 * to run and handle the interrupt. Asserting an already
+		 * pending event channel is idempotent.
+		 */
+		if (vcpu->arch.xen.timer_expires)
+			hrtimer_start_expires(&vcpu->arch.xen.timer,
+					      HRTIMER_MODE_ABS_HARD);
+
 		r = 0;
 		break;