On 09/12/2016 11:53 PM, Alex Williamson wrote:
> On Mon, 12 Sep 2016 13:19:11 +0530
> Kirti Wankhede <kwankhede@xxxxxxxxxx> 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 <kwankhede@xxxxxxxxxx> 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
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