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

[Heiner Kallweit <hkallweit1@gmail.com>]

On 04.12.2024 20:30, Alex Williamson wrote:
> On Wed, 4 Dec 2024 19:16:03 +0100
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> 
>> On Wed, Dec 04, 2024 at 06:01:36PM +0100, Heiner Kallweit wrote:
>>> 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.  
>>
>> That's odd, when this was added, why was it added this way?  The
>> class_compat stuff is for when classes move around, yet this was always
>> done in this way?
>>
>> And what tools use this symlink today that can't be updated?
> 
> It's been this way for 8 years, so it's hard to remember exact
> motivation for using this mechanism, whether we just didn't look hard
> enough at the class_compat or it didn't pass by the right set of eyes.
> Yes, it's always been this way for the mdev_bus class.
> 
> The intention here is much like other classes, that we have a node in
> sysfs where we can find devices that provide a common feature, in this
> case providing support for creating and managing vfio mediated devices
> (mdevs).  The perhaps unique part of this use case is that these devices
> aren't strictly mdev providers, they might also belong to another class
> and the mdev aspect of their behavior might be dynamically added after
> the device itself is added.
> 
> I've done some testing with this series and it does indeed seem to
> maintain compatibility with existing userspace tools, mdevctl and
> libvirt.  We can update these tools, but then we get into the breaking

Greg, is this testing, done by Alex, sufficient for you to take the series?

> userspace and deprecation period questions, where we may further delay
> removal of class_compat.  Also if we were to re-implement this, is there
> a better mechanism than proposed here within the class hierarchy, or
> would you recommend a non-class approach?  Thanks,
> 

You have /sys/bus/mdev. Couldn't we create a directory here which holds
the links to the devices in question?
Then user space would simply have to switch from /sys/class/mdev_bus
to /sys/bus/mdev/<new_dir>.

> Alex
> 
Heiner