Re: [Qemu-devel] [PATCH v7 1/4] vfio: Mediated device Core driver

[Alex Williamson]

On Mon, 19 Sep 2016 22:59:34 +0530
Kirti Wankhede <address@hidden> wrote:

> On 9/12/2016 9:23 PM, Alex Williamson wrote:
> > On Mon, 12 Sep 2016 13:19:11 +0530
> > Kirti Wankhede <address@hidden> wrote:
> >   
> >> On 9/12/2016 10:40 AM, Jike Song wrote:  
> >>> On 09/10/2016 03:55 AM, Kirti Wankhede wrote:    
> >>>> On 9/10/2016 12:12 AM, Alex Williamson wrote:    
> >>>>> On Fri, 9 Sep 2016 23:18:45 +0530
> >>>>> Kirti Wankhede <address@hidden> wrote:
> >>>>>    
> >>>>>> On 9/8/2016 1:39 PM, Jike Song wrote:    
> >>>>>>> On 08/25/2016 11:53 AM, Kirti Wankhede wrote:      
> >>>>>>    
> >>>>>>>>  +---------------+
> >>>>>>>>  |               |
> >>>>>>>>  | +-----------+ |  mdev_register_driver() +--------------+
> >>>>>>>>  | |           | +<------------------------+ __init()     |
> >>>>>>>>  | |  mdev     | |                         |              |
> >>>>>>>>  | |  bus      | +------------------------>+              |<-> VFIO 
> >>>>>>>> user
> >>>>>>>>  | |  driver   | |     probe()/remove()    | vfio_mdev.ko |    APIs
> >>>>>>>>  | |           | |                         |              |
> >>>>>>>>  | +-----------+ |                         +--------------+
> >>>>>>>>  |               |      
> >>>>>>>
> >>>>>>> This aimed to have only one single vfio bus driver for all mediated 
> >>>>>>> devices,
> >>>>>>> right?
> >>>>>>>      
> >>>>>>
> >>>>>> Yes. That's correct.
> >>>>>>
> >>>>>>    
> >>>>>>>> +
> >>>>>>>> +static int mdev_add_attribute_group(struct device *dev,
> >>>>>>>> +                                const struct attribute_group 
> >>>>>>>> **groups)
> >>>>>>>> +{
> >>>>>>>> +    return sysfs_create_groups(&dev->kobj, groups);
> >>>>>>>> +}
> >>>>>>>> +
> >>>>>>>> +static void mdev_remove_attribute_group(struct device *dev,
> >>>>>>>> +                                    const struct attribute_group 
> >>>>>>>> **groups)
> >>>>>>>> +{
> >>>>>>>> +    sysfs_remove_groups(&dev->kobj, groups);
> >>>>>>>> +}      
> >>>>>>>
> >>>>>>> These functions are not necessary. You can always specify the 
> >>>>>>> attribute groups
> >>>>>>> to dev->groups before registering a new device.
> >>>>>>>       
> >>>>>>
> >>>>>> At the time of mdev device create, I specifically didn't used
> >>>>>> dev->groups because we callback in vendor driver before that, see below
> >>>>>> code snippet, and those attributes should only be added if create()
> >>>>>> callback returns success.
> >>>>>>
> >>>>>>         ret = parent->ops->create(mdev, mdev_params);
> >>>>>>         if (ret)
> >>>>>>                 return ret;
> >>>>>>
> >>>>>>         ret = mdev_add_attribute_group(&mdev->dev,
> >>>>>>                                         parent->ops->mdev_attr_groups);
> >>>>>>         if (ret)
> >>>>>>                 parent->ops->destroy(mdev);
> >>>>>>
> >>>>>>
> >>>>>>    
> >>>>>>>> +
> >>>>>>>> +static struct parent_device *mdev_get_parent_from_dev(struct device 
> >>>>>>>> *dev)
> >>>>>>>> +{
> >>>>>>>> +    struct parent_device *parent;
> >>>>>>>> +
> >>>>>>>> +    mutex_lock(&parent_list_lock);
> >>>>>>>> +    parent = mdev_get_parent(__find_parent_device(dev));
> >>>>>>>> +    mutex_unlock(&parent_list_lock);
> >>>>>>>> +
> >>>>>>>> +    return parent;
> >>>>>>>> +}      
> >>>>>>>
> >>>>>>> As we have demonstrated, all these refs and locks and release 
> >>>>>>> workqueue are not necessary,
> >>>>>>> as long as you have an independent device associated with the mdev 
> >>>>>>> host device
> >>>>>>> ("parent" device here).
> >>>>>>>      
> >>>>>>
> >>>>>> I don't think every lock will go away with that. This also changes how
> >>>>>> mdev devices entries are created in sysfs. It adds an extra directory. 
> >>>>>>    
> >>>>>
> >>>>> Exposing the parent-child relationship through sysfs is a desirable
> >>>>> feature, so I'm not sure how this is a negative.  This part of Jike's
> >>>>> conversion was a big improvement, I thought.  Thanks,
> >>>>>    
> >>>>
> >>>> Jike's suggestion is to introduced a fake device over parent device i.e.
> >>>> mdev-host, and then all mdev devices are children of 'mdev-host' not
> >>>> children of real parent.
> >>>>    
> >>>
> >>> It really depends on how you define 'real parent' :)
> >>>
> >>> With a physical-host-mdev hierarchy, the parent of mdev devices is the 
> >>> host
> >>> device, the parent of host device is the physical device. e.g.
> >>>
> >>>         pdev            mdev_host       mdev_device
> >>>         dev<------------dev<------------dev
> >>>               parent          parent
> >>>
> >>>         Figure 1: device hierarchy
> >>>     
> >>
> >> Right, mdev-host device doesn't represent physical device nor any mdev
> >> device. Then what is the need of such device?  
> > 
> > Is there anything implicitly wrong with using a device node to host the
> > mdev child devices?  Is the argument against it only that it's
> > unnecessary?  Can we make use of the device-core parent/child
> > dependencies as Jike has done w/o that extra node?
> >  
> 
> I do feel that mdev core module would get simplified with the new sysfs
> interface without having extra node.

Can you provide an example of why that is?
 
> >>>> For example, directory structure we have now is:
> >>>> /sys/bus/pci/devices/0000\:85\:00.0/<mdev_device>
> >>>>
> >>>> mdev devices are in real parents directory.
> >>>>
> >>>> By introducing fake device it would be:
> >>>> /sys/bus/pci/devices/0000\:85\:00.0/mdev-host/<mdev_device>
> >>>>
> >>>> mdev devices are in fake device's directory.
> >>>>    
> >>>
> >>> Yes, this is the wanted directory.
> >>>     
> >>
> >> I don't think so.  
> > 
> > Why?
> >   
> 
> This directory is not mandatory. right?

Clearly you've done an implementation without it, so it's not
functionally mandatory, but Jike has made significant code reduction
and simplification with it.  Those are desirable things.

> >>>> Lock would be still required, to handle the race conditions like
> >>>> 'mdev_create' is still in process and parent device is unregistered by
> >>>> vendor driver/ parent device is unbind from vendor driver.
> >>>>    
> >>>
> >>> locks are provided to protect resources, would you elaborate more on
> >>> what is the exact resource you want to protect by a lock in mdev_create?
> >>>     
> >>
> >> Simple, in your suggestion mdev-host device. Fake device will go away if
> >> vendor driver unregisters the device from mdev module, right.  
> > 
> > I don't follow the reply here, but aiui there's ordering implicit in
> > the device core that Jike is trying to take advantage of that
> > simplifies the mdev layer significantly.  In the case of an
> > mdev_create, the device core needs to take a reference to the parent
> > object, the mdev-host I'd guess in Jike's version, the created mdev
> > device would also have a reference to that object, so the physical host
> > device could not be removed so long as there are outstanding
> > references.  It's just a matter of managing references and acquiring
> > and releasing objects.  Thanks,
> >  
> 
> I do think this could be simplified without having extra node.

The simplification is really what I'm after, whether or not it includes
an extra device node is not something I'm sure I have an opinion on
yet.  Aren't we really just talking about an extra sysfs directory
node?  Doesn't it make it easier for userspace to efficiently identify
all the mdev children when they're segregated from the other attributes
and sub-nodes of a parent device?
 
> > the created mdev
> > device would also have a reference to that object, so the physical host
> > device could not be removed so long as there are outstanding
> > references.  
> 
> Yes, this is also true when physical device is direct parent of mdev
> device. mdev device keeps reference of parent, so physical host device
> could not be removed as long as mdev devices are present. That is why
> from mdev_unregister_device() a chance is given to free all child mdev
> devices.

But why aren't we using the device core do do that?  It seems like
we're getting hung up on this device node, which is more of a stylistic
and sysfs layout issue when the primary comment is to simplify the mdev
infrastructure by taking more advantage of the parent/child
dependencies of the device core.  Thanks,

Alex