arm/arm64: KVM: Rework the arch timer to use level-triggered semantics

The arch timer currently uses edge-triggered semantics in the sense that
the line is never sampled by the vgic and lowering the line from the
timer to the vgic doesn't have any effect on the pending state of
virtual interrupts in the vgic. This means that we do not support a
guest with the otherwise valid behavior of (1) disable interrupts (2)
enable the timer (3) disable the timer (4) enable interrupts. Such a
guest would validly not expect to see any interrupts on real hardware,
but will see interrupts on KVM.

This patch fixes this shortcoming through the following series of
changes.

First, we change the flow of the timer/vgic sync/flush operations. Now
the timer is always flushed/synced before the vgic, because the vgic
samples the state of the timer output. This has the implication that we
move the timer operations in to non-preempible sections, but that is
fine after the previous commit getting rid of hrtimer schedules on every
entry/exit.

Second, we change the internal behavior of the timer, letting the timer
keep track of its previous output state, and only lower/raise the line
to the vgic when the state changes. Note that in theory this could have
been accomplished more simply by signalling the vgic every time the
state *potentially* changed, but we don't want to be hitting the vgic
more often than necessary.

Third, we get rid of the use of the map->active field in the vgic and
instead simply set the interrupt as active on the physical distributor
whenever the input to the GIC is asserted and conversely clear the
physical active state when the input to the GIC is deasserted.

Fourth, and finally, we now initialize the timer PPIs (and all the other
unused PPIs for now), to be level-triggered, and modify the sync code to
sample the line state on HW sync and re-inject a new interrupt if it is
still pending at that time.

Signed-off-by: Christoffer Dall <[e-mail]>

arch/arm/kvm/arm.c

···
 
 		if (ret <= 0 || need_new_vmid_gen(vcpu->kvm)) {
 			local_irq_enable();
+			kvm_timer_sync_hwstate(vcpu);
 			kvm_vgic_sync_hwstate(vcpu);
 			preempt_enable();
-			kvm_timer_sync_hwstate(vcpu);
 			continue;
 		}
 
···
 		kvm_guest_exit();
 		trace_kvm_exit(kvm_vcpu_trap_get_class(vcpu), *vcpu_pc(vcpu));
 
+		/*
+		 * We must sync the timer state before the vgic state so that
+		 * the vgic can properly sample the updated state of the
+		 * interrupt line.
+		 */
+		kvm_timer_sync_hwstate(vcpu);
+
 		kvm_vgic_sync_hwstate(vcpu);
 
 		preempt_enable();
-
-		kvm_timer_sync_hwstate(vcpu);
 
 		ret = handle_exit(vcpu, run, ret);
 	}

include/kvm/arm_arch_timer.h

···
 	bool				armed;
 
 	/* Timer IRQ */
-	const struct kvm_irq_level	*irq;
+	struct kvm_irq_level		irq;
 
 	/* VGIC mapping */
 	struct irq_phys_map		*map;

include/kvm/arm_vgic.h

···
 	u32			virt_irq;
 	u32			phys_irq;
 	u32			irq;
-	bool			active;
 };
 
 struct irq_phys_map_entry {
···
 struct irq_phys_map *kvm_vgic_map_phys_irq(struct kvm_vcpu *vcpu,
 					   int virt_irq, int irq);
 int kvm_vgic_unmap_phys_irq(struct kvm_vcpu *vcpu, struct irq_phys_map *map);
-bool kvm_vgic_get_phys_irq_active(struct irq_phys_map *map);
-void kvm_vgic_set_phys_irq_active(struct irq_phys_map *map, bool active);
 
 #define irqchip_in_kernel(k)	(!!((k)->arch.vgic.in_kernel))
 #define vgic_initialized(k)	(!!((k)->arch.vgic.nr_cpus))

virt/kvm/arm/arch_timer.c

···
 	}
 }
 
-static void kvm_timer_inject_irq(struct kvm_vcpu *vcpu)
-{
-	int ret;
-	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
-
-	kvm_vgic_set_phys_irq_active(timer->map, true);
-	ret = kvm_vgic_inject_mapped_irq(vcpu->kvm, vcpu->vcpu_id,
-					 timer->map,
-					 timer->irq->level);
-	WARN_ON(ret);
-}
-
 static irqreturn_t kvm_arch_timer_handler(int irq, void *dev_id)
 {
 	struct kvm_vcpu *vcpu = *(struct kvm_vcpu **)dev_id;
···
 	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
 
 	return !(timer->cntv_ctl & ARCH_TIMER_CTRL_IT_MASK) &&
-		(timer->cntv_ctl & ARCH_TIMER_CTRL_ENABLE) &&
-		!kvm_vgic_get_phys_irq_active(timer->map);
+		(timer->cntv_ctl & ARCH_TIMER_CTRL_ENABLE);
 }
 
 bool kvm_timer_should_fire(struct kvm_vcpu *vcpu)
···
 	now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
 
 	return cval <= now;
+}
+
+static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level)
+{
+	int ret;
+	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
+
+	BUG_ON(!vgic_initialized(vcpu->kvm));
+
+	timer->irq.level = new_level;
+	ret = kvm_vgic_inject_mapped_irq(vcpu->kvm, vcpu->vcpu_id,
+					 timer->map,
+					 timer->irq.level);
+	WARN_ON(ret);
+}
+
+/*
+ * Check if there was a change in the timer state (should we raise or lower
+ * the line level to the GIC).
+ */
+static void kvm_timer_update_state(struct kvm_vcpu *vcpu)
+{
+	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
+
+	/*
+	 * If userspace modified the timer registers via SET_ONE_REG before
+	 * the vgic was initialized, we mustn't set the timer->irq.level value
+	 * because the guest would never see the interrupt.  Instead wait
+	 * until we call this function from kvm_timer_flush_hwstate.
+	 */
+	if (!vgic_initialized(vcpu->kvm))
+	    return;
+
+	if (kvm_timer_should_fire(vcpu) != timer->irq.level)
+		kvm_timer_update_irq(vcpu, !timer->irq.level);
 }
 
 /*
···
 	bool phys_active;
 	int ret;
 
-	if (kvm_timer_should_fire(vcpu))
-		kvm_timer_inject_irq(vcpu);
+	kvm_timer_update_state(vcpu);
 
 	/*
-	 * We keep track of whether the edge-triggered interrupt has been
-	 * signalled to the vgic/guest, and if so, we mask the interrupt and
-	 * the physical distributor to prevent the timer from raising a
-	 * physical interrupt whenever we run a guest, preventing forward
-	 * VCPU progress.
+	 * If we enter the guest with the virtual input level to the VGIC
+	 * asserted, then we have already told the VGIC what we need to, and
+	 * we don't need to exit from the guest until the guest deactivates
+	 * the already injected interrupt, so therefore we should set the
+	 * hardware active state to prevent unnecessary exits from the guest.
+	 *
+	 * Conversely, if the virtual input level is deasserted, then always
+	 * clear the hardware active state to ensure that hardware interrupts
+	 * from the timer triggers a guest exit.
 	 */
-	if (kvm_vgic_get_phys_irq_active(timer->map))
+	if (timer->irq.level)
 		phys_active = true;
 	else
 		phys_active = false;
···
 
 	BUG_ON(timer_is_armed(timer));
 
-	if (kvm_timer_should_fire(vcpu))
-		kvm_timer_inject_irq(vcpu);
+	/*
+	 * The guest could have modified the timer registers or the timer
+	 * could have expired, update the timer state.
+	 */
+	kvm_timer_update_state(vcpu);
 }
 
 int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
···
 	 * kvm_vcpu_set_target(). To handle this, we determine
 	 * vcpu timer irq number when the vcpu is reset.
 	 */
-	timer->irq = irq;
+	timer->irq.irq = irq->irq;
 
 	/*
 	 * The bits in CNTV_CTL are architecturally reset to UNKNOWN for ARMv8
···
 	 * the ARMv7 architecture.
 	 */
 	timer->cntv_ctl = 0;
+	kvm_timer_update_state(vcpu);
 
 	/*
 	 * Tell the VGIC that the virtual interrupt is tied to a
···
 	default:
 		return -1;
 	}
+
+	kvm_timer_update_state(vcpu);
 	return 0;
 }
 

virt/kvm/arm/vgic.c

···
 	return false;
 }
 
-/*
- * If a mapped interrupt's state has been modified by the guest such that it
- * is no longer active or pending, without it have gone through the sync path,
- * then the map->active field must be cleared so the interrupt can be taken
- * again.
- */
-static void vgic_handle_clear_mapped_irq(struct kvm_vcpu *vcpu)
-{
-	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
-	struct list_head *root;
-	struct irq_phys_map_entry *entry;
-	struct irq_phys_map *map;
-
-	rcu_read_lock();
-
-	/* Check for PPIs */
-	root = &vgic_cpu->irq_phys_map_list;
-	list_for_each_entry_rcu(entry, root, entry) {
-		map = &entry->map;
-
-		if (!vgic_dist_irq_is_pending(vcpu, map->virt_irq) &&
-		    !vgic_irq_is_active(vcpu, map->virt_irq))
-			map->active = false;
-	}
-
-	rcu_read_unlock();
-}
-
 bool vgic_handle_clear_pending_reg(struct kvm *kvm,
 				   struct kvm_exit_mmio *mmio,
 				   phys_addr_t offset, int vcpu_id)
···
 					  vcpu_id, offset);
 		vgic_reg_access(mmio, reg, offset, mode);
 
-		vgic_handle_clear_mapped_irq(kvm_get_vcpu(kvm, vcpu_id));
 		vgic_update_state(kvm);
 		return true;
 	}
···
 			ACCESS_READ_VALUE | ACCESS_WRITE_CLEARBIT);
 
 	if (mmio->is_write) {
-		vgic_handle_clear_mapped_irq(kvm_get_vcpu(kvm, vcpu_id));
 		vgic_update_state(kvm);
 		return true;
 	}
···
 /*
  * Save the physical active state, and reset it to inactive.
  *
- * Return 1 if HW interrupt went from active to inactive, and 0 otherwise.
+ * Return true if there's a pending level triggered interrupt line to queue.
  */
-static int vgic_sync_hwirq(struct kvm_vcpu *vcpu, struct vgic_lr vlr)
+static bool vgic_sync_hwirq(struct kvm_vcpu *vcpu, int lr, struct vgic_lr vlr)
 {
+	struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
 	struct irq_phys_map *map;
+	bool phys_active;
+	bool level_pending;
 	int ret;
 
 	if (!(vlr.state & LR_HW))
-		return 0;
+		return false;
 
 	map = vgic_irq_map_search(vcpu, vlr.irq);
 	BUG_ON(!map);
 
 	ret = irq_get_irqchip_state(map->irq,
 				    IRQCHIP_STATE_ACTIVE,
-				    &map->active);
+				    &phys_active);
 
 	WARN_ON(ret);
 
-	if (map->active)
+	if (phys_active)
 		return 0;
 
-	return 1;
+	/* Mapped edge-triggered interrupts not yet supported. */
+	WARN_ON(vgic_irq_is_edge(vcpu, vlr.irq));
+	spin_lock(&dist->lock);
+	level_pending = process_level_irq(vcpu, lr, vlr);
+	spin_unlock(&dist->lock);
+	return level_pending;
 }
 
 /* Sync back the VGIC state after a guest run */
···
 			continue;
 
 		vlr = vgic_get_lr(vcpu, lr);
-		if (vgic_sync_hwirq(vcpu, vlr)) {
-			/*
-			 * So this is a HW interrupt that the guest
-			 * EOI-ed. Clean the LR state and allow the
-			 * interrupt to be sampled again.
-			 */
-			vlr.state = 0;
-			vlr.hwirq = 0;
-			vgic_set_lr(vcpu, lr, vlr);
-			vgic_irq_clear_queued(vcpu, vlr.irq);
-			set_bit(lr, elrsr_ptr);
-		}
+		if (vgic_sync_hwirq(vcpu, lr, vlr))
+			level_pending = true;
 
 		if (!test_bit(lr, elrsr_ptr))
 			continue;
···
 }
 
 /**
- * kvm_vgic_get_phys_irq_active - Return the active state of a mapped IRQ
- *
- * Return the logical active state of a mapped interrupt. This doesn't
- * necessarily reflects the current HW state.
- */
-bool kvm_vgic_get_phys_irq_active(struct irq_phys_map *map)
-{
-	BUG_ON(!map);
-	return map->active;
-}
-
-/**
- * kvm_vgic_set_phys_irq_active - Set the active state of a mapped IRQ
- *
- * Set the logical active state of a mapped interrupt. This doesn't
- * immediately affects the HW state.
- */
-void kvm_vgic_set_phys_irq_active(struct irq_phys_map *map, bool active)
-{
-	BUG_ON(!map);
-	map->active = active;
-}
-
-/**
  * kvm_vgic_unmap_phys_irq - Remove a virtual to physical IRQ mapping
  * @vcpu: The VCPU pointer
  * @map: The pointer to a mapping obtained through kvm_vgic_map_phys_irq
···
 		}
 
 		/*
-		 * Enable all SGIs and configure all private IRQs as
-		 * edge-triggered.
+		 * Enable and configure all SGIs to be edge-triggere and
+		 * configure all PPIs as level-triggered.
 		 */
 		for (i = 0; i < VGIC_NR_PRIVATE_IRQS; i++) {
-			if (i < VGIC_NR_SGIS)
+			if (i < VGIC_NR_SGIS) {
+				/* SGIs */
 				vgic_bitmap_set_irq_val(&dist->irq_enabled,
 							vcpu->vcpu_id, i, 1);
-			if (i < VGIC_NR_PRIVATE_IRQS)
 				vgic_bitmap_set_irq_val(&dist->irq_cfg,
 							vcpu->vcpu_id, i,
 							VGIC_CFG_EDGE);
+			} else if (i < VGIC_NR_PRIVATE_IRQS) {
+				/* PPIs */
+				vgic_bitmap_set_irq_val(&dist->irq_cfg,
+							vcpu->vcpu_id, i,
+							VGIC_CFG_LEVEL);
+			}
 		}
 
 		vgic_enable(vcpu);