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

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

On Fri, Dec 06, 2024 at 08:35:47AM +0100, Heiner Kallweit wrote:
> 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?

Were devices actually removed from the system and all worked well?

> > 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?

Links to devices is not what class links are for, so why is this in
/sys/class/ at all?

> Then user space would simply have to switch from /sys/class/mdev_bus
> to /sys/bus/mdev/<new_dir>.

I think you are confusing what /sys/class/ is for here, if you have
devices on a "bus" then they need to be in /sys/bus/   class has nothing
to do with that.

So can we just drop the /sys/class/ mistake entirely?

thanks,

greg k-h