vfio: Remove vfio_group dev_counter

This counts the number of devices attached to a vfio_group, ie the number
of items in the group->device_list.

It is only read in vfio_pin_pages(), as some kind of protection against
limitations in type1.

However, with all the code cleanups in this area, now that
vfio_pin_pages() accepts a vfio_device directly it is redundant.  All
drivers are already calling vfio_register_emulated_iommu_dev() which
directly creates a group specifically for the device and thus it is
guaranteed that there is a singleton group.

Leave a note in the comment about this requirement and remove the logic.

Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/0-v2-d4374a7bf0c9+c4-vfio_dev_counter_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_main.c

@@ -75,7 +75,6 @@ struct vfio_group {
 	struct list_head		vfio_next;
 	struct list_head		container_next;
 	enum vfio_group_type		type;
-	unsigned int			dev_counter;
 	struct rw_semaphore		group_rwsem;
 	struct kvm			*kvm;
 	struct file			*opened_file;
@@ -609,7 +608,6 @@ static int __vfio_register_dev(struct vfio_device *device,
 
 	mutex_lock(&group->device_lock);
 	list_add(&device->group_next, &group->device_list);
-	group->dev_counter++;
 	mutex_unlock(&group->device_lock);
 
 	return 0;
@@ -697,7 +695,6 @@ void vfio_unregister_group_dev(struct vfio_device *device)
 
 	mutex_lock(&group->device_lock);
 	list_del(&device->group_next);
-	group->dev_counter--;
 	mutex_unlock(&group->device_lock);
 
 	if (group->type == VFIO_NO_IOMMU || group->type == VFIO_EMULATED_IOMMU)
@@ -1991,6 +1988,9 @@ EXPORT_SYMBOL(vfio_set_irqs_validate_and_prepare);
  * @prot [in]    : protection flags
  * @pages[out]   : array of host pages
  * Return error or number of pages pinned.
+ *
+ * A driver may only call this function if the vfio_device was created
+ * by vfio_register_emulated_iommu_dev().
  */
 int vfio_pin_pages(struct vfio_device *device, dma_addr_t iova,
 		   int npage, int prot, struct page **pages)
@@ -2006,9 +2006,6 @@ int vfio_pin_pages(struct vfio_device *device, dma_addr_t iova,
 	if (npage > VFIO_PIN_PAGES_MAX_ENTRIES)
 		return -E2BIG;
 
-	if (group->dev_counter > 1)
-		return -EINVAL;
-
 	/* group->container cannot change while a vfio device is open */
 	container = group->container;
 	driver = container->iommu_driver;