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

[Jike Song]

On 09/12/2016 11:53 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?
>  
>>>> 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?
> 
>>>> 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,

Hi Kirti/Neo,

Any thought on this part? Could we have more discussion in case that
further concern raised?


--
Thanks,
Jike