vfio-mdev: Wire in a request handler for mdev parent

While performing some destructive tests with vfio-ccw, where the
paths to a device are forcible removed and thus the device itself
is unreachable, it is rather easy to end up in an endless loop in
vfio_del_group_dev() due to the lack of a request callback for the
associated device.

In this example, one MDEV (77c) is used by a guest, while another
(77b) is not. The symptom is that the iommu is detached from the
mdev for 77b, but not 77c, until that guest is shutdown:

    [  238.794867] vfio_ccw 0.0.077b: MDEV: Unregistering
    [  238.794996] vfio_mdev 11f2d2bc-4083-431d-a023-eff72715c4f0: Removing from iommu group 2
    [  238.795001] vfio_mdev 11f2d2bc-4083-431d-a023-eff72715c4f0: MDEV: detaching iommu
    [  238.795036] vfio_ccw 0.0.077c: MDEV: Unregistering
    ...silence...

Let's wire in the request call back to the mdev device, so that a
device being physically removed from the host can be (gracefully?)
handled by the parent device at the time the device is removed.

Add a message when registering the device if a driver doesn't
provide this callback, so a clue is given that this same loop
may be encountered in a similar situation, and a message when
this occurs instead of the awkward silence noted above.
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/mdev/mdev_core.c

@@ -154,6 +154,10 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
 	if (!dev)
 		return -EINVAL;
 
+	/* Not mandatory, but its absence could be a problem */
+	if (!ops->request)
+		dev_info(dev, "Driver cannot be asked to release device\n");
+
 	mutex_lock(&parent_list_lock);
 
 	/* Check for duplicate */

drivers/vfio/mdev/vfio_mdev.c

@@ -98,6 +98,18 @@ static int vfio_mdev_mmap(void *device_data, struct vm_area_struct *vma)
 	return parent->ops->mmap(mdev, vma);
 }
 
+static void vfio_mdev_request(void *device_data, unsigned int count)
+{
+	struct mdev_device *mdev = device_data;
+	struct mdev_parent *parent = mdev->parent;
+
+	if (parent->ops->request)
+		parent->ops->request(mdev, count);
+	else if (count == 0)
+		dev_notice(mdev_dev(mdev),
+			   "No mdev vendor driver request callback support, blocked until released by user\n");
+}
+
 static const struct vfio_device_ops vfio_mdev_dev_ops = {
 	.name		= "vfio-mdev",
 	.open		= vfio_mdev_open,
@@ -106,6 +118,7 @@ static const struct vfio_device_ops vfio_mdev_dev_ops = {
 	.read		= vfio_mdev_read,
 	.write		= vfio_mdev_write,
 	.mmap		= vfio_mdev_mmap,
+	.request	= vfio_mdev_request,
 };
 
 static int vfio_mdev_probe(struct device *dev)

include/linux/mdev.h

@@ -72,6 +72,9 @@ struct device *mdev_get_iommu_device(struct device *dev);
  * @mmap:		mmap callback
  *			@mdev: mediated device structure
  *			@vma: vma structure
+ * @request:		request callback to release device
+ *			@mdev: mediated device structure
+ *			@count: request sequence number
  * Parent device that support mediated device should be registered with mdev
  * module with mdev_parent_ops structure.
  **/
@@ -92,6 +95,7 @@ struct mdev_parent_ops {
 	long	(*ioctl)(struct mdev_device *mdev, unsigned int cmd,
 			 unsigned long arg);
 	int	(*mmap)(struct mdev_device *mdev, struct vm_area_struct *vma);
+	void	(*request)(struct mdev_device *mdev, unsigned int count);
 };
 
 /* interface for exporting mdev supported type attributes */