vfio: Export the device set open count

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2155664
Conflicts: Context, container code has not yet been split out via
           cdc71fe ("vfio: Move container code into drivers/vfio/container.c")

commit 5cd189e
Author: Anthony DeRossi <ajderossi@gmail.com>
Date:   Wed Nov 9 17:40:26 2022 -0800

    vfio: Export the device set open count

    The open count of a device set is the sum of the open counts of all
    devices in the set. Drivers can use this value to determine whether
    shared resources are in use without tracking them manually or accessing
    the private open_count in vfio_device.

    Signed-off-by: Anthony DeRossi <ajderossi@gmail.com>
    Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
    Reviewed-by: Kevin Tian <kevin.tian@intel.com>
    Reviewed-by: Yi Liu <yi.l.liu@intel.com>
    Link: https://lore.kernel.org/r/20221110014027.28780-3-ajderossi@gmail.com
    Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: Alex Williamson

drivers/vfio/vfio_main.c

@@ -302,6 +302,19 @@ static void vfio_container_put(struct vfio_container *container)
 	kref_put(&container->kref, vfio_container_release);
 }
 
+unsigned int vfio_device_set_open_count(struct vfio_device_set *dev_set)
+{
+	struct vfio_device *cur;
+	unsigned int open_count = 0;
+
+	lockdep_assert_held(&dev_set->lock);
+
+	list_for_each_entry(cur, &dev_set->device_list, dev_set_list)
+		open_count += cur->open_count;
+	return open_count;
+}
+EXPORT_SYMBOL_GPL(vfio_device_set_open_count);
+
 /*
  * Group objects - create, release, get, put, search
  */

include/linux/vfio.h

@@ -145,6 +145,7 @@ int vfio_register_emulated_iommu_dev(struct vfio_device *device);
 void vfio_unregister_group_dev(struct vfio_device *device);
 
 int vfio_assign_device_set(struct vfio_device *device, void *set_id);
+unsigned int vfio_device_set_open_count(struct vfio_device_set *dev_set);
 
 int vfio_mig_get_next_state(struct vfio_device *device,
 			    enum vfio_device_mig_state cur_fsm,