KVM: x86: Introduce a workqueue to deliver PIT timer interrupts

We really want to "kvm_set_irq" during the hrtimer callback,
but that is risky because that is during interrupt context.
Instead, offload the work to a workqueue, which is a bit safer
and should provide most of the same functionality.
Signed-off-by: Chris Lalancette <clalance@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/i8254.c

@@ -34,6 +34,7 @@
 
 #include <linux/kvm_host.h>
 #include <linux/slab.h>
+#include <linux/workqueue.h>
 
 #include "irq.h"
 #include "i8254.h"
@@ -244,11 +245,22 @@ static void kvm_pit_ack_irq(struct kvm_irq_ack_notifier *kian)
 {
 	struct kvm_kpit_state *ps = container_of(kian, struct kvm_kpit_state,
 						 irq_ack_notifier);
-	raw_spin_lock(&ps->inject_lock);
-	if (atomic_dec_return(&ps->pit_timer.pending) < 0)
+	int value;
+
+	spin_lock(&ps->inject_lock);
+	value = atomic_dec_return(&ps->pit_timer.pending);
+	if (value < 0)
+		/* spurious acks can be generated if, for example, the
+		 * PIC is being reset.  Handle it gracefully here
+		 */
 		atomic_inc(&ps->pit_timer.pending);
+	else if (value > 0)
+		/* in this case, we had multiple outstanding pit interrupts
+		 * that we needed to inject.  Reinject
+		 */
+		queue_work(ps->pit->wq, &ps->pit->expired);
 	ps->irq_ack = 1;
-	raw_spin_unlock(&ps->inject_lock);
+	spin_unlock(&ps->inject_lock);
 }
 
 void __kvm_migrate_pit_timer(struct kvm_vcpu *vcpu)
@@ -264,10 +276,10 @@ void __kvm_migrate_pit_timer(struct kvm_vcpu *vcpu)
 		hrtimer_start_expires(timer, HRTIMER_MODE_ABS);
 }
 
-static void destroy_pit_timer(struct kvm_timer *pt)
+static void destroy_pit_timer(struct kvm_pit *pit)
 {
-	pr_debug("execute del timer!\n");
-	hrtimer_cancel(&pt->timer);
+	hrtimer_cancel(&pit->pit_state.pit_timer.timer);
+	cancel_work_sync(&pit->expired);
 }
 
 static bool kpit_is_periodic(struct kvm_timer *ktimer)
@@ -281,6 +293,60 @@ static struct kvm_timer_ops kpit_ops = {
 	.is_periodic = kpit_is_periodic,
 };
 
+static void pit_do_work(struct work_struct *work)
+{
+	struct kvm_pit *pit = container_of(work, struct kvm_pit, expired);
+	struct kvm *kvm = pit->kvm;
+	struct kvm_vcpu *vcpu;
+	int i;
+	struct kvm_kpit_state *ps = &pit->pit_state;
+	int inject = 0;
+
+	/* Try to inject pending interrupts when
+	 * last one has been acked.
+	 */
+	spin_lock(&ps->inject_lock);
+	if (ps->irq_ack) {
+		ps->irq_ack = 0;
+		inject = 1;
+	}
+	spin_unlock(&ps->inject_lock);
+	if (inject) {
+		kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1);
+		kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 0);
+
+		/*
+		 * Provides NMI watchdog support via Virtual Wire mode.
+		 * The route is: PIT -> PIC -> LVT0 in NMI mode.
+		 *
+		 * Note: Our Virtual Wire implementation is simplified, only
+		 * propagating PIT interrupts to all VCPUs when they have set
+		 * LVT0 to NMI delivery. Other PIC interrupts are just sent to
+		 * VCPU0, and only if its LVT0 is in EXTINT mode.
+		 */
+		if (kvm->arch.vapics_in_nmi_mode > 0)
+			kvm_for_each_vcpu(i, vcpu, kvm)
+				kvm_apic_nmi_wd_deliver(vcpu);
+	}
+}
+
+static enum hrtimer_restart pit_timer_fn(struct hrtimer *data)
+{
+	struct kvm_timer *ktimer = container_of(data, struct kvm_timer, timer);
+	struct kvm_pit *pt = ktimer->kvm->arch.vpit;
+
+	if (ktimer->reinject || !atomic_read(&ktimer->pending)) {
+		atomic_inc(&ktimer->pending);
+		queue_work(pt->wq, &pt->expired);
+	}
+
+	if (ktimer->t_ops->is_periodic(ktimer)) {
+		hrtimer_add_expires_ns(&ktimer->timer, ktimer->period);
+		return HRTIMER_RESTART;
+	} else
+		return HRTIMER_NORESTART;
+}
+
 static void create_pit_timer(struct kvm_kpit_state *ps, u32 val, int is_period)
 {
 	struct kvm_timer *pt = &ps->pit_timer;
@@ -292,13 +358,13 @@ static void create_pit_timer(struct kvm_kpit_state *ps, u32 val, int is_period)
 
 	/* TODO The new value only affected after the retriggered */
 	hrtimer_cancel(&pt->timer);
+	cancel_work_sync(&ps->pit->expired);
 	pt->period = interval;
 	ps->is_periodic = is_period;
 
-	pt->timer.function = kvm_timer_fn;
+	pt->timer.function = pit_timer_fn;
 	pt->t_ops = &kpit_ops;
 	pt->kvm = ps->pit->kvm;
-	pt->vcpu = pt->kvm->bsp_vcpu;
 
 	atomic_set(&pt->pending, 0);
 	ps->irq_ack = 1;
@@ -347,7 +413,7 @@ static void pit_load_count(struct kvm *kvm, int channel, u32 val)
 		}
 		break;
 	default:
-		destroy_pit_timer(&ps->pit_timer);
+		destroy_pit_timer(kvm->arch.vpit);
 	}
 }
 
@@ -626,7 +692,14 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
 
 	mutex_init(&pit->pit_state.lock);
 	mutex_lock(&pit->pit_state.lock);
-	raw_spin_lock_init(&pit->pit_state.inject_lock);
+	spin_lock_init(&pit->pit_state.inject_lock);
+
+	pit->wq = create_singlethread_workqueue("kvm-pit-wq");
+	if (!pit->wq) {
+		kfree(pit);
+		return NULL;
+	}
+	INIT_WORK(&pit->expired, pit_do_work);
 
 	kvm->arch.vpit = pit;
 	pit->kvm = kvm;
@@ -685,54 +758,10 @@ void kvm_free_pit(struct kvm *kvm)
 		mutex_lock(&kvm->arch.vpit->pit_state.lock);
 		timer = &kvm->arch.vpit->pit_state.pit_timer.timer;
 		hrtimer_cancel(timer);
+		cancel_work_sync(&kvm->arch.vpit->expired);
 		kvm_free_irq_source_id(kvm, kvm->arch.vpit->irq_source_id);
 		mutex_unlock(&kvm->arch.vpit->pit_state.lock);
+		destroy_workqueue(kvm->arch.vpit->wq);
 		kfree(kvm->arch.vpit);
 	}
 }
-
-static void __inject_pit_timer_intr(struct kvm *kvm)
-{
-	struct kvm_vcpu *vcpu;
-	int i;
-
-	kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1);
-	kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 0);
-
-	/*
-	 * Provides NMI watchdog support via Virtual Wire mode.
-	 * The route is: PIT -> PIC -> LVT0 in NMI mode.
-	 *
-	 * Note: Our Virtual Wire implementation is simplified, only
-	 * propagating PIT interrupts to all VCPUs when they have set
-	 * LVT0 to NMI delivery. Other PIC interrupts are just sent to
-	 * VCPU0, and only if its LVT0 is in EXTINT mode.
-	 */
-	if (kvm->arch.vapics_in_nmi_mode > 0)
-		kvm_for_each_vcpu(i, vcpu, kvm)
-			kvm_apic_nmi_wd_deliver(vcpu);
-}
-
-void kvm_inject_pit_timer_irqs(struct kvm_vcpu *vcpu)
-{
-	struct kvm_pit *pit = vcpu->kvm->arch.vpit;
-	struct kvm *kvm = vcpu->kvm;
-	struct kvm_kpit_state *ps;
-
-	if (pit) {
-		int inject = 0;
-		ps = &pit->pit_state;
-
-		/* Try to inject pending interrupts when
-		 * last one has been acked.
-		 */
-		raw_spin_lock(&ps->inject_lock);
-		if (atomic_read(&ps->pit_timer.pending) && ps->irq_ack) {
-			ps->irq_ack = 0;
-			inject = 1;
-		}
-		raw_spin_unlock(&ps->inject_lock);
-		if (inject)
-			__inject_pit_timer_intr(kvm);
-	}
-}

arch/x86/kvm/i8254.h

@@ -27,7 +27,7 @@ struct kvm_kpit_state {
 	u32    speaker_data_on;
 	struct mutex lock;
 	struct kvm_pit *pit;
-	raw_spinlock_t inject_lock;
+	spinlock_t inject_lock;
 	unsigned long irq_ack;
 	struct kvm_irq_ack_notifier irq_ack_notifier;
 };
@@ -40,6 +40,8 @@ struct kvm_pit {
 	struct kvm_kpit_state pit_state;
 	int irq_source_id;
 	struct kvm_irq_mask_notifier mask_notifier;
+	struct workqueue_struct *wq;
+	struct work_struct expired;
 };
 
 #define KVM_PIT_BASE_ADDRESS	    0x40

arch/x86/kvm/irq.c

@@ -90,7 +90,6 @@ EXPORT_SYMBOL_GPL(kvm_cpu_get_interrupt);
 void kvm_inject_pending_timer_irqs(struct kvm_vcpu *vcpu)
 {
 	kvm_inject_apic_timer_irqs(vcpu);
-	kvm_inject_pit_timer_irqs(vcpu);
 	/* TODO: PIT, RTC etc. */
 }
 EXPORT_SYMBOL_GPL(kvm_inject_pending_timer_irqs);