irqbypass: Use xarray to track producers and consumers

Track IRQ bypass producers and consumers using an xarray to avoid the O(2n)
insertion time associated with walking a list to check for duplicate
entries, and to search for an partner.

At low (tens or few hundreds) total producer/consumer counts, using a list
is faster due to the need to allocate backing storage for xarray.  But as
count creeps into the thousands, xarray wins easily, and can provide
several orders of magnitude better latency at high counts.  E.g. hundreds
of nanoseconds vs. hundreds of milliseconds.

Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: David Matlack <dmatlack@google.com>
Cc: Like Xu <like.xu.linux@gmail.com>
Cc: Binbin Wu <binbin.wu@linux.intel.com>
Reported-by: Yong He <alexyonghe@tencent.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217379
Link: https://lore.kernel.org/all/20230801115646.33990-1-likexu@tencent.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20250516230734.2564775-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

include/linux/irqbypass.h

@@ -33,7 +33,6 @@ struct irq_bypass_consumer;
 
 /**
  * struct irq_bypass_producer - IRQ bypass producer definition
- * @node: IRQ bypass manager private list management
  * @eventfd: eventfd context used to match producers and consumers
  * @consumer: The connected consumer (NULL if no connection)
  * @irq: Linux IRQ number for the producer device
@@ -47,7 +46,6 @@ struct irq_bypass_consumer;
  * for a physical device assigned to a VM.
  */
 struct irq_bypass_producer {
-	struct list_head node;
 	struct eventfd_ctx *eventfd;
 	struct irq_bypass_consumer *consumer;
 	int irq;
@@ -61,7 +59,6 @@ struct irq_bypass_producer {
 
 /**
  * struct irq_bypass_consumer - IRQ bypass consumer definition
- * @node: IRQ bypass manager private list management
  * @eventfd: eventfd context used to match producers and consumers
  * @producer: The connected producer (NULL if no connection)
  * @add_producer: Connect the IRQ consumer to an IRQ producer
@@ -75,7 +72,6 @@ struct irq_bypass_producer {
  * portions of the interrupt handling to the VM.
  */
 struct irq_bypass_consumer {
-	struct list_head node;
 	struct eventfd_ctx *eventfd;
 	struct irq_bypass_producer *producer;
 

virt/lib/irqbypass.c

@@ -22,8 +22,8 @@
 MODULE_LICENSE("GPL v2");
 MODULE_DESCRIPTION("IRQ bypass manager utility module");
 
-static LIST_HEAD(producers);
-static LIST_HEAD(consumers);
+static DEFINE_XARRAY(producers);
+static DEFINE_XARRAY(consumers);
 static DEFINE_MUTEX(lock);
 
 /* @lock must be held when calling connect */
@@ -86,13 +86,13 @@ static void __disconnect(struct irq_bypass_producer *prod,
  * @producer: pointer to producer structure
  * @eventfd: pointer to the eventfd context associated with the producer
  *
- * Add the provided IRQ producer to the list of producers and connect
- * with any matching eventfd found on the IRQ consumers list.
+ * Add the provided IRQ producer to the set of producers and connect with the
+ * consumer with a matching eventfd, if one exists.
  */
 int irq_bypass_register_producer(struct irq_bypass_producer *producer,
 				 struct eventfd_ctx *eventfd)
 {
-	struct irq_bypass_producer *tmp;
+	unsigned long index = (unsigned long)eventfd;
 	struct irq_bypass_consumer *consumer;
 	int ret;
 
@@ -101,22 +101,20 @@ int irq_bypass_register_producer(struct irq_bypass_producer *producer,
 
 	guard(mutex)(&lock);
 
-	list_for_each_entry(tmp, &producers, node) {
-		if (tmp->eventfd == eventfd)
-			return -EBUSY;
-	}
+	ret = xa_insert(&producers, index, producer, GFP_KERNEL);
+	if (ret)
+		return ret;
 
-	list_for_each_entry(consumer, &consumers, node) {
-		if (consumer->eventfd == eventfd) {
-			ret = __connect(producer, consumer);
-			if (ret)
-				return ret;
-			break;
+	consumer = xa_load(&consumers, index);
+	if (consumer) {
+		ret = __connect(producer, consumer);
+		if (ret) {
+			WARN_ON_ONCE(xa_erase(&producers, index) != producer);
+			return ret;
 		}
 	}
 
 	producer->eventfd = eventfd;
-	list_add(&producer->node, &producers);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(irq_bypass_register_producer);
@@ -125,11 +123,14 @@ EXPORT_SYMBOL_GPL(irq_bypass_register_producer);
  * irq_bypass_unregister_producer - unregister IRQ bypass producer
  * @producer: pointer to producer structure
  *
- * Remove a previously registered IRQ producer from the list of producers
- * and disconnect it from any connected IRQ consumer.
+ * Remove a previously registered IRQ producer (note, it's safe to call this
+ * even if registration was unsuccessful).  Disconnect from the associated
+ * consumer, if one exists.
  */
 void irq_bypass_unregister_producer(struct irq_bypass_producer *producer)
 {
+	unsigned long index = (unsigned long)producer->eventfd;
+
 	if (!producer->eventfd)
 		return;
 
@@ -138,8 +139,8 @@ void irq_bypass_unregister_producer(struct irq_bypass_producer *producer)
 	if (producer->consumer)
 		__disconnect(producer, producer->consumer);
 
+	WARN_ON_ONCE(xa_erase(&producers, index) != producer);
 	producer->eventfd = NULL;
-	list_del(&producer->node);
 }
 EXPORT_SYMBOL_GPL(irq_bypass_unregister_producer);
 
@@ -148,13 +149,13 @@ EXPORT_SYMBOL_GPL(irq_bypass_unregister_producer);
  * @consumer: pointer to consumer structure
  * @eventfd: pointer to the eventfd context associated with the consumer
  *
- * Add the provided IRQ consumer to the list of consumers and connect
- * with any matching eventfd found on the IRQ producer list.
+ * Add the provided IRQ consumer to the set of consumers and connect with the
+ * producer with a matching eventfd, if one exists.
  */
 int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer,
 				 struct eventfd_ctx *eventfd)
 {
-	struct irq_bypass_consumer *tmp;
+	unsigned long index = (unsigned long)eventfd;
 	struct irq_bypass_producer *producer;
 	int ret;
 
@@ -166,22 +167,20 @@ int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer,
 
 	guard(mutex)(&lock);
 
-	list_for_each_entry(tmp, &consumers, node) {
-		if (tmp->eventfd == eventfd)
-			return -EBUSY;
-	}
+	ret = xa_insert(&consumers, index, consumer, GFP_KERNEL);
+	if (ret)
+		return ret;
 
-	list_for_each_entry(producer, &producers, node) {
-		if (producer->eventfd == eventfd) {
-			ret = __connect(producer, consumer);
-			if (ret)
-				return ret;
-			break;
+	producer = xa_load(&producers, index);
+	if (producer) {
+		ret = __connect(producer, consumer);
+		if (ret) {
+			WARN_ON_ONCE(xa_erase(&consumers, index) != consumer);
+			return ret;
 		}
 	}
 
 	consumer->eventfd = eventfd;
-	list_add(&consumer->node, &consumers);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(irq_bypass_register_consumer);
@@ -190,11 +189,14 @@ EXPORT_SYMBOL_GPL(irq_bypass_register_consumer);
  * irq_bypass_unregister_consumer - unregister IRQ bypass consumer
  * @consumer: pointer to consumer structure
  *
- * Remove a previously registered IRQ consumer from the list of consumers
- * and disconnect it from any connected IRQ producer.
+ * Remove a previously registered IRQ consumer (note, it's safe to call this
+ * even if registration was unsuccessful).  Disconnect from the associated
+ * producer, if one exists.
  */
 void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
 {
+	unsigned long index = (unsigned long)consumer->eventfd;
+
 	if (!consumer->eventfd)
 		return;
 
@@ -203,7 +205,7 @@ void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
 	if (consumer->producer)
 		__disconnect(consumer->producer, consumer);
 
+	WARN_ON_ONCE(xa_erase(&consumers, index) != consumer);
 	consumer->eventfd = NULL;
-	list_del(&consumer->node);
 }
 EXPORT_SYMBOL_GPL(irq_bypass_unregister_consumer);