Re: [PATCH v2 1/1] virtio: add driver_override support

[Max Gurtovoy]

On 09/12/2025 18:34, Michael S. Tsirkin wrote:

On Tue, Dec 09, 2025 at 12:09:08AM +0200, Max Gurtovoy wrote:

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.

oh, it's a device driver not the bus driver, actually.

Yes. I'll fix the commit message.

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/.

what is this "mydriver" though? what are valid examples that
we want to support?

This is an example for a custom virtio block driver.

It can be any custom virtio-XX driver (XX=FS/NET/..).

Signed-off-by: Avraham Evdaev <aevd...@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurto...@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