Add support for the 'driver_override' attribute to Virtio devices. This
allows users to control which Virtio bus driver binds to a given Virtio
device.

If 'driver_override' is not set, the existing behavior is preserved and
devices will continue to auto-bind to the first matching Virtio bus
driver.

Tested with virtio blk device (virtio core and pci drivers are loaded):

  $ modprobe my_virtio_blk

  # automatically unbind from virtio_blk driver and override + bind to
  # my_virtio_blk driver.
  $ driverctl -v -b virtio set-override virtio0 my_virtio_blk

In addition, driverctl saves the configuration persistently under
/etc/driverctl.d/.

Signed-off-by: Avraham Evdaev <aevdaev@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
---

changes from v1:
 - use !strcmp() to compare strings (MST)
 - extend commit msg with example (MST)

---
 drivers/virtio/virtio.c | 34 ++++++++++++++++++++++++++++++++++
 include/linux/virtio.h  |  4 ++++
 2 files changed, 38 insertions(+)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index a09eb4d62f82..993dc928be49 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -61,12 +61,41 @@ static ssize_t features_show(struct device *_d,
 }
 static DEVICE_ATTR_RO(features);
 
+static ssize_t driver_override_store(struct device *_d,
+				     struct device_attribute *attr,
+				     const char *buf, size_t count)
+{
+	struct virtio_device *dev = dev_to_virtio(_d);
+	int ret;
+
+	ret = driver_set_override(_d, &dev->driver_override, buf, count);
+	if (ret)
+		return ret;
+
+	return count;
+}
+
+static ssize_t driver_override_show(struct device *_d,
+				    struct device_attribute *attr, char *buf)
+{
+	struct virtio_device *dev = dev_to_virtio(_d);
+	ssize_t len;
+
+	device_lock(_d);
+	len = sysfs_emit(buf, "%s\n", dev->driver_override);
+	device_unlock(_d);
+
+	return len;
+}
+static DEVICE_ATTR_RW(driver_override);
+
 static struct attribute *virtio_dev_attrs[] = {
 	&dev_attr_device.attr,
 	&dev_attr_vendor.attr,
 	&dev_attr_status.attr,
 	&dev_attr_modalias.attr,
 	&dev_attr_features.attr,
+	&dev_attr_driver_override.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(virtio_dev);
@@ -88,6 +117,10 @@ static int virtio_dev_match(struct device *_dv, const struct device_driver *_dr)
 	struct virtio_device *dev = dev_to_virtio(_dv);
 	const struct virtio_device_id *ids;
 
+	/* Check override first, and if set, only use the named driver */
+	if (dev->driver_override)
+		return !strcmp(dev->driver_override, _dr->name);
+
 	ids = drv_to_virtio(_dr)->id_table;
 	for (i = 0; ids[i].device; i++)
 		if (virtio_id_match(dev, &ids[i]))
@@ -582,6 +615,7 @@ void unregister_virtio_device(struct virtio_device *dev)
 {
 	int index = dev->index; /* save for after device release */
 
+	kfree(dev->driver_override);
 	device_unregister(&dev->dev);
 	virtio_debug_device_exit(dev);
 	ida_free(&virtio_index_ida, index);
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index db31fc6f4f1f..418bb490bdc6 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -138,6 +138,9 @@ struct virtio_admin_cmd {
  * @config_lock: protects configuration change reporting
  * @vqs_list_lock: protects @vqs.
  * @dev: underlying device.
+ * @driver_override: driver name to force a match; do not set directly,
+ *                   because core frees it; use driver_set_override() to
+ *                   set or clear it.
  * @id: the device type identification (used to match it with a driver).
  * @config: the configuration ops for this device.
  * @vringh_config: configuration ops for host vrings.
@@ -158,6 +161,7 @@ struct virtio_device {
 	spinlock_t config_lock;
 	spinlock_t vqs_list_lock;
 	struct device dev;
+	const char *driver_override;
 	struct virtio_device_id id;
 	const struct virtio_config_ops *config;
 	const struct vringh_config_ops *vringh_config;
-- 
2.18.1