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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

> 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? 

> 
> 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