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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 09/12/2025 19:48, Michael S. Tsirkin wrote:
> On Tue, Dec 09, 2025 at 07:45:19PM +0200, Max Gurtovoy wrote:
>> 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/..).
> Is "custom" a way to say "out of tree"?

The driver_override mechanism fits both in-tree/out-of-tree drivers.

>
>>>> 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