Using a vfio device, a notifier block can be registered to receive
select device events.  Notifiers can only be registered for contained
devices, ie. they are available through a user context.  Registration
of a notifier increments the reference to that container context
therefore notifiers must minimally respond to the release event by
asynchronously removing notifiers.

Signed-off-by: Alex Williamson <redacted>
 drivers/vfio/Kconfig |    1 +
 drivers/vfio/vfio.c  |   35 +++++++++++++++++++++++++++++++++++
 include/linux/vfio.h |    9 +++++++++
 3 files changed, 45 insertions(+)

diff --git a/drivers/vfio/Kconfig b/drivers/vfio/Kconfig
index 90c0525b1e0c..9a67675c9b6c 100644
+++ b/drivers/vfio/Kconfig

@@ -23,6 +23,7 @@ menuconfig VFIO
 	tristate "VFIO Non-Privileged userspace driver framework"
 	select IOMMU_API
 	select VFIO_IOMMU_TYPE1 if (X86 || S390 || ARM || ARM64)
+	select SRCU
 	help
 	  VFIO provides a framework for secure userspace device drivers.
 	  See Documentation/driver-api/vfio.rst for more details.

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index c47895539a1a..7f6d00e54e83 100644
+++ b/drivers/vfio/vfio.c

@@ -105,6 +105,7 @@ struct vfio_device {
 	struct list_head		group_next;
 	void				*device_data;
 	struct inode			*inode;
+	struct srcu_notifier_head	notifier;
 };
 
 #ifdef CONFIG_VFIO_NOIOMMU

@@ -601,6 +602,7 @@ struct vfio_device *vfio_group_create_device(struct vfio_group *group,
 	device->ops = ops;
 	device->device_data = device_data;
 	dev_set_drvdata(dev, device);
+	srcu_init_notifier_head(&device->notifier);
 
 	/* No need to get group_lock, caller has group reference */
 	vfio_group_get(group);

@@ -1785,6 +1787,39 @@ static const struct file_operations vfio_device_fops = {
 	.mmap		= vfio_device_fops_mmap,
 };
 
+int vfio_device_register_notifier(struct vfio_device *device,
+				  struct notifier_block *nb)
+{
+	int ret;
+
+	/* Container ref persists until unregister on success */
+	ret =  vfio_group_add_container_user(device->group);