[RFC PATCH] irq: IRQ bypass manager

[Alex Williamson]

When a physical I/O device is assigned to a virtual machine through
facilities like VFIO and KVM, the interrupt for the device generally
bounces through the host system before being injected into the VM.
However, hardware technologies exist that often allow the host to be
bypassed for some of these scenarios.  Intel Posted Interrupts allow
the specified physical edge interrupts to be directly injected into a
guest when delivered to a physical processor while the vCPU is
running.  ARM IRQ Forwarding allows the hypervisor to handle level
triggered device interrupts as edge interrupts, by giving the guest
control of de-asserting and unmasking the interrupt line.

The IRQ bypass manager here is meant to provide the shim to connect
interrupt producers, generally the host physical device driver, with
interrupt consumers, generally the hypervisor, in order to configure
these bypass mechanism.  To do this, we base the connection on a
shared, opaque token.  For KVM-VFIO this is expected to be an
eventfd_ctx since this is the connection we already use to connect an
eventfd to an irqfd on the in-kernel path.  When a producer and
consumer with matching tokens is found, callbacks via both registered
participants allow the bypass facilities to be automatically enabled.

Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Cc: Eric Auger <eric.auger@xxxxxxxxxx>
---

This is the current draft of the IRQ bypass manager, I've made the
following changes:

 - Incorporated Eric's extensions (I would welcome Sign-offs from all
   involved in the development, especially Eric - I've gone ahead and
   added Linaro copyright for the contributions so far)
 - Module support with module reference tracking
 - might_sleep() as suggested by Paolo
 - kerneldoc as suggested by Paolo
 - Renamed file s/bypass/irqbypass/ because a module named "bypass"
   is strange

Issues:
 - The update() callback is defined but not used
 - We can't have *all* the callbacks be optional.  I assume add/del
   are required
 - Naming consistency, stop is to start as suspend is to resume, not
   stop/resume
 - Callback descriptions including why we need separate stop/start
   hooks when it seems like the callee could reasonably assume such
   around the add/del callbacks
 - Need functional prototypes for both PI and forwarding

 include/linux/irqbypass.h |   75 ++++++++++++++++
 kernel/irq/Kconfig        |    3 +
 kernel/irq/Makefile       |    1 
 kernel/irq/irqbypass.c    |  206 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 285 insertions(+)
 create mode 100644 include/linux/irqbypass.h
 create mode 100644 kernel/irq/irqbypass.c

diff --git a/include/linux/irqbypass.h b/include/linux/irqbypass.h
new file mode 100644
index 0000000..cc7ce45
--- /dev/null
+++ b/include/linux/irqbypass.h
@@ -0,0 +1,75 @@
+/*
+ * IRQ offload/bypass manager
+ *
+ * Copyright (C) 2015 Red Hat, Inc.
+ * Copyright (c) 2015 Linaro Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+#ifndef IRQBYPASS_H
+#define IRQBYPASS_H
+
+#include <linux/list.h>
+
+struct irq_bypass_consumer;
+
+/**
+ * struct irq_bypass_producer - IRQ bypass producer definition
+ * @node: IRQ bypass manager private list management
+ * @token: opaque token to match between producer and consumer
+ * @irq: Linux IRQ number for the producer device
+ * @stop:
+ * @resume:
+ * @add_consumer:
+ * @del_consumer:
+ *
+ * The IRQ bypass producer structure represents an interrupt source for
+ * participation in possible host bypass, for instance an interrupt vector
+ * for a physical device assigned to a VM.
+ */
+struct irq_bypass_producer {
+	struct list_head node;
+	void *token;
+	int irq; /* linux irq */
+	void (*stop)(struct irq_bypass_producer *);
+	void (*resume)(struct irq_bypass_producer *);
+	void (*add_consumer)(struct irq_bypass_producer *,
+			     struct irq_bypass_consumer *);
+	void (*del_consumer)(struct irq_bypass_producer *,
+			     struct irq_bypass_consumer *);
+};
+
+/**
+ * struct irq_bypass_consumer - IRQ bypass consumer definition
+ * @node: IRQ bypass manager private list management
+ * @token: opaque token to match between producer and consumer
+ * @stop:
+ * @resume:
+ * @add_consumer:
+ * @del_consumer:
+ * @update:
+ *
+ * The IRQ bypass consumer structure represents an interrupt sink for
+ * participation in possible host bypass, for instance a hypervisor may
+ * support offloads to allow bypassing the host entirely or offload
+ * portions of the interrupt handling to the VM.
+ */
+struct irq_bypass_consumer {
+	struct list_head node;
+	void *token;
+	void (*stop)(struct irq_bypass_consumer *);
+	void (*resume)(struct irq_bypass_consumer *);
+	void (*add_producer)(struct irq_bypass_consumer *,
+			     struct irq_bypass_producer *);
+	void (*del_producer)(struct irq_bypass_consumer *,
+			     struct irq_bypass_producer *);
+	void (*update)(struct irq_bypass_consumer *);
+};
+
+int irq_bypass_register_producer(struct irq_bypass_producer *);
+void irq_bypass_unregister_producer(struct irq_bypass_producer *);
+int irq_bypass_register_consumer(struct irq_bypass_consumer *);
+void irq_bypass_unregister_consumer(struct irq_bypass_consumer *);
+#endif /* IRQBYPASS_H */
diff --git a/kernel/irq/Kconfig b/kernel/irq/Kconfig
index 9a76e3b..de5afe3 100644
--- a/kernel/irq/Kconfig
+++ b/kernel/irq/Kconfig
@@ -100,4 +100,7 @@ config SPARSE_IRQ
 
 	  If you don't know what to do here, say N.
 
+config IRQ_BYPASS_MANAGER
+	tristate
+
 endmenu
diff --git a/kernel/irq/Makefile b/kernel/irq/Makefile
index d121235..a6cfec7 100644
--- a/kernel/irq/Makefile
+++ b/kernel/irq/Makefile
@@ -7,3 +7,4 @@ obj-$(CONFIG_PROC_FS) += proc.o
 obj-$(CONFIG_GENERIC_PENDING_IRQ) += migration.o
 obj-$(CONFIG_PM_SLEEP) += pm.o
 obj-$(CONFIG_GENERIC_MSI_IRQ) += msi.o
+obj-$(CONFIG_IRQ_BYPASS_MANAGER) += irqbypass.o
diff --git a/kernel/irq/irqbypass.c b/kernel/irq/irqbypass.c
new file mode 100644
index 0000000..3fd828d
--- /dev/null
+++ b/kernel/irq/irqbypass.c
@@ -0,0 +1,206 @@
+/*
+ * IRQ offload/bypass manager
+ *
+ * Copyright (C) 2015 Red Hat, Inc.
+ * Copyright (c) 2015 Linaro Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * Various virtualization hardware acceleration techniques allow bypassing
+ * or offloading interrupts received from devices around the host kernel.
+ * Posted Interrupts on Intel VT-d systems can allow interrupts to be
+ * received directly by a virtual machine.  ARM IRQ Forwarding can allow
+ * level triggered device interrupts to be de-asserted directly by the VM.
+ * This manager allows interrupt producers and consumers to find each other
+ * to enable this sort of bypass.
+ */
+
+#include <linux/irqbypass.h>
+#include <linux/list.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+
+MODULE_LICENSE("GPL v2");
+MODULE_DESCRIPTION("IRQ bypass manager utility module");
+
+static LIST_HEAD(producers);
+static LIST_HEAD(consumers);
+static DEFINE_MUTEX(lock);
+
+/* @lock must be held when calling connect */
+static void __connect(struct irq_bypass_producer *prod,
+		      struct irq_bypass_consumer *cons)
+{
+	if (prod->stop)
+		prod->stop(prod);
+	if (cons->stop)
+		cons->stop(cons);
+	if (prod->add_consumer)
+		prod->add_consumer(prod, cons);
+	if (cons->add_producer)
+		cons->add_producer(cons, prod);
+	if (cons->resume)
+		cons->resume(cons);
+	if (prod->resume)
+		prod->resume(prod);
+}
+
+/* @lock must be held when calling disconnect */
+static void __disconnect(struct irq_bypass_producer *prod,
+			 struct irq_bypass_consumer *cons)
+{
+	if (prod->stop)
+		prod->stop(prod);
+	if (cons->stop)
+		cons->stop(cons);
+	if (cons->del_producer)
+		cons->del_producer(cons, prod);
+	if (prod->del_consumer)
+		prod->del_consumer(prod, cons);
+	if (cons->resume)
+		cons->resume(cons);
+	if (prod->resume)
+		prod->resume(prod);
+}
+
+/**
+ * irq_bypass_register_producer - register IRQ bypass producer
+ * @producer: pointer to producer structure
+ *
+ * Add the provided IRQ producer to the list of producers and connect
+ * with any matching tokens found on the IRQ consumers list.
+ */
+int irq_bypass_register_producer(struct irq_bypass_producer *producer)
+{
+	struct irq_bypass_producer *tmp;
+	struct irq_bypass_consumer *consumer;
+
+	might_sleep();
+
+	if (!try_module_get(THIS_MODULE))
+		return -ENODEV;
+
+	mutex_lock(&lock);
+
+	list_for_each_entry(tmp, &producers, node) {
+		if (tmp->token == producer->token) {
+			mutex_unlock(&lock);
+			module_put(THIS_MODULE);
+			return -EINVAL;
+		}
+	}
+
+	list_add(&producer->node, &producers);
+
+	list_for_each_entry(consumer, &consumers, node) {
+		if (consumer->token == producer->token) {
+			__connect(producer, consumer);
+			break;
+		}
+	}
+
+	mutex_unlock(&lock);
+	return 0;
+}
+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 disconnected from any connected IRQ consumers.
+ */
+void irq_bypass_unregister_producer(struct irq_bypass_producer *producer)
+{
+	struct irq_bypass_consumer *consumer;
+
+	might_sleep();
+
+	mutex_lock(&lock);
+
+	list_for_each_entry(consumer, &consumers, node) {
+		if (consumer->token == producer->token) {
+			__disconnect(producer, consumer);
+			break;
+		}
+	}
+
+	list_del(&producer->node);
+
+	mutex_unlock(&lock);
+	module_put(THIS_MODULE);
+}
+EXPORT_SYMBOL_GPL(irq_bypass_unregister_producer);
+
+/**
+ * irq_bypass_register_consumer - register IRQ bypass consumer
+ * @consumer: pointer to consumer structure
+ *
+ * Add the provided IRQ consumer to the list of consumers and connect
+ * with any matching tokens found on the IRQ producer list.
+ */
+int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer)
+{
+	struct irq_bypass_consumer *tmp;
+	struct irq_bypass_producer *producer;
+
+	might_sleep();
+
+	if (!try_module_get(THIS_MODULE))
+		return -ENODEV;
+
+	mutex_lock(&lock);
+
+	list_for_each_entry(tmp, &consumers, node) {
+		if (tmp->token == consumer->token) {
+			mutex_unlock(&lock);
+			module_put(THIS_MODULE);
+			return -EINVAL;
+		}
+	}
+
+	list_add(&consumer->node, &consumers);
+
+	list_for_each_entry(producer, &producers, node) {
+		if (producer->token == consumer->token) {
+			__connect(producer, consumer);
+			break;
+		}
+	}
+
+	mutex_unlock(&lock);
+	return 0;
+}
+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 disconnected from any connected IRQ producers.
+ */
+void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
+{
+	struct irq_bypass_producer *producer;
+
+	might_sleep();
+
+	mutex_lock(&lock);
+
+	list_for_each_entry(producer, &producers, node) {
+		if (producer->token == consumer->token) {
+			__disconnect(producer, consumer);
+			break;
+		}
+	}
+
+	list_del(&consumer->node);
+
+	mutex_unlock(&lock);
+	module_put(THIS_MODULE);
+}
+EXPORT_SYMBOL_GPL(irq_bypass_unregister_consumer);

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