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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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?

thanks,

greg k-h