[RFC 09/17] bypass: IRQ bypass manager proto by Alex

[Eric Auger]

From: Alex Williamson <alex.williamson@xxxxxxxxxx>

There are plenty of details to be filled in, but I think the basics
looks something like the code below.  The IRQ bypass manager just
defines a pair of structures, one for interrupt producers and one for
interrupt consumers.  I'm certain that we'll need more callbacks than
I've defined below, but figuring out what those should be for the best
abstraction is the hardest part of this idea.  The manager provides both
registration and de-registration interfaces for both types of objects
and keeps lists for each, protected by a lock.  The manager doesn't even
really need to know what the match token is, but I assume for our
purposes it will be an eventfd_ctx.

On the vfio side, the producer struct would be embedded in the
vfio_pci_irq_ctx struct.  KVM would probably embed the consumer struct
in _irqfd.  As I've coded below, the IRQ bypass manager calls the
consumer callbacks, so the producer struct would need fields or
callbacks to provide the consumer the info it needs.  AIUI the Posted
Interrupt model, VFIO only needs to provide data to the consumer.  For
IRQ Forwarding, I think the producer needs to be informed when bypass is
active to model the incoming interrupt as edge vs level.

I've prototyped the base IRQ bypass manager here as static, but I don't
see any reason it couldn't be a module that's loaded by dependency when
either vfio-pci or kvm-intel is loaded (or other producer/consumer
objects).

Is this a reasonable starting point to craft the additional fields and
callbacks and interaction of who calls who that we need to support
Posted Interrupts and IRQ Forwarding?  Is the AMD version of this still
alive?  Thanks,

Alex
---
 arch/x86/kvm/Kconfig              |   1 +
 drivers/vfio/pci/Kconfig          |   1 +
 drivers/vfio/pci/vfio_pci_intrs.c |   6 ++
 include/linux/irqbypass.h         |  23 ++++++++
 kernel/irq/Kconfig                |   3 +
 kernel/irq/Makefile               |   1 +
 kernel/irq/bypass.c               | 116 ++++++++++++++++++++++++++++++++++++++
 virt/kvm/eventfd.c                |   4 ++
 8 files changed, 155 insertions(+)
 create mode 100644 include/linux/irqbypass.h
 create mode 100644 kernel/irq/bypass.c

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index d8a1d56..86d0d77 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -61,6 +61,7 @@ config KVM_INTEL
 	depends on KVM
 	# for perf_guest_get_msrs():
 	depends on CPU_SUP_INTEL
+	select IRQ_BYPASS_MANAGER
 	---help---
 	  Provides support for KVM on Intel processors equipped with the VT
 	  extensions.
diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
index 579d83b..02912f1 100644
--- a/drivers/vfio/pci/Kconfig
+++ b/drivers/vfio/pci/Kconfig
@@ -2,6 +2,7 @@ config VFIO_PCI
 	tristate "VFIO support for PCI devices"
 	depends on VFIO && PCI && EVENTFD
 	select VFIO_VIRQFD
+	select IRQ_BYPASS_MANAGER
 	help
 	  Support for the PCI VFIO bus driver.  This is required to make
 	  use of PCI drivers using the VFIO framework.
diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
index 1f577b4..4e053be 100644
--- a/drivers/vfio/pci/vfio_pci_intrs.c
+++ b/drivers/vfio/pci/vfio_pci_intrs.c
@@ -181,6 +181,7 @@ static int vfio_intx_set_signal(struct vfio_pci_device *vdev, int fd)
 
 	if (vdev->ctx[0].trigger) {
 		free_irq(pdev->irq, vdev);
+		/* irq_bypass_unregister_producer(); */
 		kfree(vdev->ctx[0].name);
 		eventfd_ctx_put(vdev->ctx[0].trigger);
 		vdev->ctx[0].trigger = NULL;
@@ -214,6 +215,8 @@ static int vfio_intx_set_signal(struct vfio_pci_device *vdev, int fd)
 		return ret;
 	}
 
+	/* irq_bypass_register_producer(); */
+
 	/*
 	 * INTx disable will stick across the new irq setup,
 	 * disable_irq won't.
@@ -319,6 +322,7 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_device *vdev,
 
 	if (vdev->ctx[vector].trigger) {
 		free_irq(irq, vdev->ctx[vector].trigger);
+		/* irq_bypass_unregister_producer(); */
 		kfree(vdev->ctx[vector].name);
 		eventfd_ctx_put(vdev->ctx[vector].trigger);
 		vdev->ctx[vector].trigger = NULL;
@@ -360,6 +364,8 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_device *vdev,
 		return ret;
 	}
 
+	/* irq_bypass_register_producer(); */
+
 	vdev->ctx[vector].trigger = trigger;
 
 	return 0;
diff --git a/include/linux/irqbypass.h b/include/linux/irqbypass.h
new file mode 100644
index 0000000..718508e
--- /dev/null
+++ b/include/linux/irqbypass.h
@@ -0,0 +1,23 @@
+#ifndef IRQBYPASS_H
+#define IRQBYPASS_H
+
+#include <linux/list.h>
+
+struct irq_bypass_producer {
+	struct list_head node;
+	void *token;
+	/* TBD */
+};
+
+struct irq_bypass_consumer {
+	struct list_head node;
+	void *token;
+	void (*add_producer)(struct irq_bypass_producer *);
+	void (*del_producer)(struct irq_bypass_producer *);
+};
+
+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..4502cdc 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
+	bool
+
 endmenu
diff --git a/kernel/irq/Makefile b/kernel/irq/Makefile
index d121235..a30ed77 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) += bypass.o
diff --git a/kernel/irq/bypass.c b/kernel/irq/bypass.c
new file mode 100644
index 0000000..5d0f92b
--- /dev/null
+++ b/kernel/irq/bypass.c
@@ -0,0 +1,116 @@
+/*
+ * IRQ offload/bypass manager
+ *
+ * Various virtualization hardware acceleration techniques allow bypassing
+ * or offloading interrupts receieved from devices around the host kernel.
+ * Posted Interrupts on Intel VT-d systems can allow interrupts to be
+ * recieved 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>
+
+static LIST_HEAD(producers);
+static LIST_HEAD(consumers);
+static DEFINE_MUTEX(lock);
+
+int irq_bypass_register_producer(struct irq_bypass_producer *producer)
+{
+	struct irq_bypass_producer *tmp;
+	struct irq_bypass_consumer *consumer;
+	int ret = 0;
+
+	mutex_lock(&lock);
+
+	list_for_each_entry(tmp, &producers, node) {
+		if (tmp->token == producer->token) {
+			ret = -EINVAL;
+			goto unlock;
+		}
+	}
+
+	list_add(&producer->node, &producers);
+
+	list_for_each_entry(consumer, &consumers, node) {
+		if (consumer->token == producer->token) {
+			consumer->add_producer(producer);
+			break;
+		}
+	}
+unlock:
+	mutex_unlock(&lock);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(irq_bypass_register_producer);
+
+void irq_bypass_unregister_producer(struct irq_bypass_producer *producer)
+{
+	struct irq_bypass_consumer *consumer;
+
+	mutex_lock(&lock);
+
+	list_for_each_entry(consumer, &consumers, node) {
+		if (consumer->token == producer->token) {
+			consumer->del_producer(producer);
+			break;
+		}
+	}
+
+	list_del(&producer->node);
+
+	mutex_unlock(&lock);
+}
+EXPORT_SYMBOL_GPL(irq_bypass_unregister_producer);
+
+int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer)
+{
+	struct irq_bypass_consumer *tmp;
+	struct irq_bypass_producer *producer;
+	int ret = 0;
+
+	mutex_lock(&lock);
+
+	list_for_each_entry(tmp, &consumers, node) {
+		if (tmp->token == consumer->token) {
+			ret = -EINVAL;
+			goto unlock;
+		}
+	}
+
+	list_add(&consumer->node, &consumers);
+
+	list_for_each_entry(producer, &producers, node) {
+		if (producer->token == consumer->token) {
+			consumer->add_producer(producer);
+			break;
+		}
+	}
+unlock:
+	mutex_unlock(&lock);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(irq_bypass_register_consumer);
+
+void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
+{
+	struct irq_bypass_producer *producer;
+
+	mutex_lock(&lock);
+
+	list_for_each_entry(producer, &producers, node) {
+		if (producer->token == consumer->token) {
+			consumer->del_producer(producer);
+			break;
+		}
+	}
+
+	list_del(&consumer->node);
+
+	mutex_unlock(&lock);
+}
+EXPORT_SYMBOL_GPL(irq_bypass_unregister_consumer);
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 9ff4193..f3da161 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -429,6 +429,8 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 	 */
 	fdput(f);
 
+	/* irq_bypass_register_consumer(); */
+
 	return 0;
 
 fail:
@@ -528,6 +530,8 @@ kvm_irqfd_deassign(struct kvm *kvm, struct kvm_irqfd *args)
 	struct _irqfd *irqfd, *tmp;
 	struct eventfd_ctx *eventfd;
 
+	/* irq_bypass_unregister_consumer() */
+
 	eventfd = eventfd_ctx_fdget(args->fd);
 	if (IS_ERR(eventfd))
 		return PTR_ERR(eventfd);
-- 
1.9.1

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