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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

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 <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