s390/vfio-ap: Driver feature advertisement

Advertise features of the driver for the benefit of automated tooling
like Libvirt and mdevctl.

Signed-off-by: Jason J. Herne <jjherne@linux.ibm.com>
Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Link: https://lore.kernel.org/r/20240916120123.11484-1-jjherne@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: Jason J. Herne

Documentation/arch/s390/vfio-ap.rst

@@ -999,6 +999,36 @@ the vfio_ap mediated device to which it is assigned as long as each new APQN
 resulting from plugging it in references a queue device bound to the vfio_ap
 device driver.
 
+Driver Features
+===============
+The vfio_ap driver exposes a sysfs file containing supported features.
+This exists so third party tools (like Libvirt and mdevctl) can query the
+availability of specific features.
+
+The features list can be found here: /sys/bus/matrix/devices/matrix/features
+
+Entries are space delimited. Each entry consists of a combination of
+alphanumeric and underscore characters.
+
+Example:
+cat /sys/bus/matrix/devices/matrix/features
+guest_matrix dyn ap_config
+
+the following features are advertised:
+
+---------------+---------------------------------------------------------------+
+| Flag         | Description                                                   |
++==============+===============================================================+
+| guest_matrix | guest_matrix attribute exists. It reports the matrix of       |
+|              | adapters and domains that are or will be passed through to a  |
+|              | guest when the mdev is attached to it.                        |
++--------------+---------------------------------------------------------------+
+| dyn          | Indicates hot plug/unplug of AP adapters, domains and control |
+|              | domains for a guest to which the mdev is attached.            |
++------------+-----------------------------------------------------------------+
+| ap_config    | ap_config interface for one-shot modifications to mdev config |
++--------------+---------------------------------------------------------------+
+
 Limitations
 ===========
 Live guest migration is not supported for guests using AP devices without

drivers/s390/crypto/vfio_ap_drv.c

@@ -26,6 +26,18 @@ MODULE_LICENSE("GPL v2");
 struct ap_matrix_dev *matrix_dev;
 debug_info_t *vfio_ap_dbf_info;
 
+static ssize_t features_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "guest_matrix hotplug ap_config\n");
+}
+static DEVICE_ATTR_RO(features);
+
+static struct attribute *matrix_dev_attrs[] = {
+	&dev_attr_features.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(matrix_dev);
+
 /* Only type 10 adapters (CEX4 and later) are supported
  * by the AP matrix device driver
  */
@@ -68,6 +80,7 @@ static struct device_driver matrix_driver = {
 	.name = "vfio_ap",
 	.bus = &matrix_bus,
 	.suppress_bind_attrs = true,
+	.dev_groups = matrix_dev_groups,
 };
 
 static int vfio_ap_matrix_dev_create(void)