[PATCHv0 RFC] kvm: irqfd support for level interrupts

["Michael S. Tsirkin" <mst@redhat.com>]

Here's an untested patch with partial support for level triggered
interrupts in irqfd. What this patch has: support for clearing interrupt
on ack. What this patch does not have: support signalling eventfd on ack
so that userspace can take action and e.g. reenable interrupt.

Gleb, Marcelo, I'd like your input on the approach taken wrt locking.
Does it look sane?

Avi, how's the interface? I intend to also add an eventfd probably in
the padding in the irqfd struct.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---

diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index 230a91a..8bf16af 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -488,6 +488,7 @@ struct kvm_x86_mce {
 #endif
 
 #define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)
+#define KVM_IRQFD_FLAG_LEVEL (1 << 1)
 
 struct kvm_irqfd {
 	__u32 fd;
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 99017e8..fcbf5b5 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -45,12 +45,14 @@ struct _irqfd {
 	struct kvm               *kvm;
 	struct eventfd_ctx       *eventfd;
 	int                       gsi;
+	int                       is_level;
 	struct list_head          list;
 	poll_table                pt;
 	wait_queue_head_t        *wqh;
 	wait_queue_t              wait;
 	struct work_struct        inject;
 	struct work_struct        shutdown;
+	struct kvm_irq_ack_notifier kian;
 };
 
 static struct workqueue_struct *irqfd_cleanup_wq;
@@ -63,10 +65,15 @@ irqfd_inject(struct work_struct *work)
 
 	mutex_lock(&kvm->irq_lock);
 	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
-	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
+	if (!irqfd->is_level)
+		kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
 	mutex_unlock(&kvm->irq_lock);
 }
 
+static void irqfd_irq_acked(struct kvm_irq_ack_notifier *kian)
+{
+	kvm_set_irq(kian->kvm, KVM_USERSPACE_IRQ_SOURCE_ID, kian->gsi, 0);
+}
 /*
  * Race-free decouple logic (ordering is critical)
  */
@@ -87,6 +94,9 @@ irqfd_shutdown(struct work_struct *work)
 	 */
 	flush_work(&irqfd->inject);
 
+	if (irqfd->is_level)
+		kvm_unregister_irq_ack_notifier(&irqfd->kian);
+
 	/*
 	 * It is now safe to release the object's resources
 	 */
@@ -166,7 +176,7 @@ irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
 }
 
 static int
-kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
+kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi, int is_level)
 {
 	struct _irqfd *irqfd;
 	struct file *file = NULL;
@@ -180,6 +190,7 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
 
 	irqfd->kvm = kvm;
 	irqfd->gsi = gsi;
+	irqfd->is_level = is_level;
 	INIT_LIST_HEAD(&irqfd->list);
 	INIT_WORK(&irqfd->inject, irqfd_inject);
 	INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
@@ -198,6 +209,12 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
 
 	irqfd->eventfd = eventfd;
 
+	if (is_level) {
+		irqfd->kian.gsi = gsi;
+		irqfd->kian.irq_acked = irqfd_irq_acked;
+		kvm_register_irq_ack_notifier(&irqfd->kian);
+	}
+
 	/*
 	 * Install our own custom wake-up handling so we are notified via
 	 * a callback whenever someone signals the underlying eventfd
@@ -281,10 +298,13 @@ kvm_irqfd_deassign(struct kvm *kvm, int fd, int gsi)
 int
 kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
 {
+	if (flags & ~(KVM_IRQFD_FLAG_DEASSIGN | KVM_IRQFD_FLAG_LEVEL))
+		return -EINVAL;
+
 	if (flags & KVM_IRQFD_FLAG_DEASSIGN)
 		return kvm_irqfd_deassign(kvm, fd, gsi);
 
-	return kvm_irqfd_assign(kvm, fd, gsi);
+	return kvm_irqfd_assign(kvm, fd, gsi, !!(flags & KVM_IRQFD_FLAG_LEVEL));
 }
 
 /*