Re: [PATCH 2/3] vfio/mdev: inline needed class_compat functionality

[Heiner Kallweit <hkallweit1@gmail.com>]

On 04.12.2024 10:32, Greg Kroah-Hartman wrote:
> On Tue, Dec 03, 2024 at 09:11:47PM +0100, Heiner Kallweit wrote:
>> vfio/mdev is the last user of class_compat, and it doesn't use it for
>> the intended purpose. See kdoc of class_compat_register():
>> Compatibility class are meant as a temporary user-space compatibility
>> workaround when converting a family of class devices to a bus devices.
> 
> True, so waht is mdev doing here?
> 
>> In addition it uses only a part of the class_compat functionality.
>> So inline the needed functionality, and afterwards all class_compat
>> code can be removed.
>>
>> No functional change intended. Compile-tested only.
>>
>> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
>> ---
>>  drivers/vfio/mdev/mdev_core.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
>> index ed4737de4..a22c49804 100644
>> --- a/drivers/vfio/mdev/mdev_core.c
>> +++ b/drivers/vfio/mdev/mdev_core.c
>> @@ -18,7 +18,7 @@
>>  #define DRIVER_AUTHOR		"NVIDIA Corporation"
>>  #define DRIVER_DESC		"Mediated device Core Driver"
>>  
>> -static struct class_compat *mdev_bus_compat_class;
>> +static struct kobject *mdev_bus_kobj;
> 
> 
> 
>>  
>>  static LIST_HEAD(mdev_list);
>>  static DEFINE_MUTEX(mdev_list_lock);
>> @@ -76,7 +76,7 @@ int mdev_register_parent(struct mdev_parent *parent, struct device *dev,
>>  	if (ret)
>>  		return ret;
>>  
>> -	ret = class_compat_create_link(mdev_bus_compat_class, dev, NULL);
>> +	ret = sysfs_create_link(mdev_bus_kobj, &dev->kobj, dev_name(dev));
> 
> This feels really wrong, why create a link to a random kobject?  Who is
> using this kobject link?
> 
>>  	if (ret)
>>  		dev_warn(dev, "Failed to create compatibility class link\n");
>>  
>> @@ -98,7 +98,7 @@ void mdev_unregister_parent(struct mdev_parent *parent)
>>  	dev_info(parent->dev, "MDEV: Unregistering\n");
>>  
>>  	down_write(&parent->unreg_sem);
>> -	class_compat_remove_link(mdev_bus_compat_class, parent->dev, NULL);
>> +	sysfs_remove_link(mdev_bus_kobj, dev_name(parent->dev));
>>  	device_for_each_child(parent->dev, NULL, mdev_device_remove_cb);
>>  	parent_remove_sysfs_files(parent);
>>  	up_write(&parent->unreg_sem);
>> @@ -251,8 +251,8 @@ static int __init mdev_init(void)
>>  	if (ret)
>>  		return ret;
>>  
>> -	mdev_bus_compat_class = class_compat_register("mdev_bus");
>> -	if (!mdev_bus_compat_class) {
>> +	mdev_bus_kobj = class_pseudo_register("mdev_bus");
> 
> But this isn't a class, so let's not fake it please.  Let's fix this
> properly, odds are all of this code can just be removed entirely, right?
> 

After I removed class_compat from i2c core, I asked Alex basically the
same thing: whether class_compat support can be removed from vfio/mdev too.

His reply:
I'm afraid we have active userspace tools dependent on
/sys/class/mdev_bus currently, libvirt for one.  We link mdev parent
devices here and I believe it's the only way for userspace to find
those parent devices registered for creating mdev devices.  If there's a
desire to remove class_compat, we might need to add some mdev
infrastructure to register the class ourselves to maintain the parent
links.


It's my understanding that /sys/class/mdev_bus has nothing in common
with an actual class, it's just a container for devices which at least
partially belong to other classes. And there's user space tools depending
on this structure.

> thanks,
> 
> greg k-h