vdpa: add driver_override support

`driver_override` allows to control which of the vDPA bus drivers
binds to a vDPA device.

If `driver_override` is not set, the previous behaviour is followed:
devices use the first vDPA bus driver loaded (unless auto binding
is disabled).

Tested on Fedora 34 with driverctl(8):
  $ modprobe virtio-vdpa
  $ modprobe vhost-vdpa
  $ modprobe vdpa-sim-net

  $ vdpa dev add mgmtdev vdpasim_net name dev1

  # dev1 is attached to the first vDPA bus driver loaded
  $ driverctl -b vdpa list-devices
    dev1 virtio_vdpa

  $ driverctl -b vdpa set-override dev1 vhost_vdpa

  $ driverctl -b vdpa list-devices
    dev1 vhost_vdpa [*]

  Note: driverctl(8) integrates with udev so the binding is
  preserved.

Suggested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211126164753.181829-3-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-vdpa b/Documentation/ABI/testing/sysfs-bus-vdpa
index 4e55761a39df..28a6111202ba 100644
--- a/Documentation/ABI/testing/sysfs-bus-vdpa
+++ b/Documentation/ABI/testing/sysfs-bus-vdpa
@@ -35,3 +35,23 @@ Description:
 		Writing a device name to this file will cause the driver to
 		attempt to unbind from the device. This may be useful when
 		overriding default bindings.
+
+What:		/sys/bus/vdpa/devices/.../driver_override
+Date:		November 2021
+Contact:	virtualization@lists.linux-foundation.org
+Description:
+		This file allows the driver for a device to be specified.
+		When specified, only a driver with a name matching the value
+		written to driver_override will have an opportunity to bind to
+		the device. The override is specified by writing a string to the
+		driver_override file (echo vhost-vdpa > driver_override) and may
+		be cleared with an empty string (echo > driver_override).
+		This returns the device to standard matching rules binding.
+		Writing to driver_override does not automatically unbind the
+		device from its current driver or make any attempt to
+		automatically load the specified driver. If no driver with a
+		matching name is currently loaded in the kernel, the device will
+		not bind to any driver. This also allows devices to opt-out of
+		driver binding using a driver_override name such as "none".
+		Only a single driver may be specified in the override, there is
+		no support for parsing delimiters.