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Subject: Re: [PATCH v5 05/13] s390: vfio-ap: register matrix device with VFIO
 mdev framework
To:     Cornelia Huck <cohuck@redhat.com>
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From:   Tony Krowiak <akrowiak@linux.vnet.ibm.com>
Date:   Tue, 22 May 2018 17:41:19 -0400
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On 05/22/2018 04:19 AM, Cornelia Huck wrote:
> On Mon, 21 May 2018 11:13:58 -0400
> Tony Krowiak <akrowiak@linux.vnet.ibm.com> wrote:
>
>> On 05/17/2018 03:44 AM, Cornelia Huck wrote:
>>> On Mon, 14 May 2018 15:42:18 -0400
>>> Tony Krowiak <akrowiak@linux.vnet.ibm.com> wrote:
>>>   
>>>> On 05/11/2018 01:18 PM, Halil Pasic wrote:
>>>>> On 05/07/2018 05:11 PM, Tony Krowiak wrote:
>>>>>> Registers the matrix device created by the VFIO AP device
>>>>>> driver with the VFIO mediated device framework.
>>>>>> Registering the matrix device will create the sysfs
>>>>>> structures needed to create mediated matrix devices
>>>>>> each of which will be used to configure the AP matrix
>>>>>> for a guest and connect it to the VFIO AP device driver.
>>>>>> +static int vfio_ap_mdev_create(struct kobject *kobj, struct
>>>>>> mdev_device *mdev)
>>>>>> +{
>>>>>> +    struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev));
>>>>>> +
>>>>>> +    ap_matrix->available_instances--;
>>>>>> +
>>>>>> +    return 0;
>>>>>> +}
>>>>>> +
>>>>>> +static int vfio_ap_mdev_remove(struct mdev_device *mdev)
>>>>>> +{
>>>>>> +    struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev));
>>>>>> +
>>>>>> +    ap_matrix->available_instances++;
>>>>>> +
>>>>>> +    return 0;
>>>>>> +}
>>>>>> +
>>>>> The above functions seem to be called with the lock of this
>>>>> auto-generated
>>>>> mdev parent device held. That's why we don't have to care about
>>>>> synchronization
>>>>> ourselves, right?
>>>> I would assume as much. The comments for the 'struct mdev_parent_ops' in
>>>> include/linux/mdev.h do not mention anything about synchronization, nor
>>>> did I
>>>> see any locking or synchronization in the vfio_ccw implementation after
>>>> which
>>>> I modeled my code, so frankly it is something I did not consider.
>>>>   
>>>>> A small comment in the code could be helpful for mdev non-experts.
>>>>> Hell, I would
>>>>> even consider documenting it for all mdev -- took me some time to
>>>>> figure out.
>>>> You may want to bring this up with the VFIO mdev maintainers, but I'd be
>>>> happy to
>>>> include a comment in the functions in question if you think it important.
>>> Important note: There's currently a patch on list that removes the mdev
>>> parent mutex, and it seems there was never intended to be any
>>> serialization in that place by the mdev core. (Look for "vfio/mdev:
>>> Check globally for duplicate devices".)
>> The patch on the list holds the mdev_list_lock during create and remove
>> of an mdev device, so it looks like no synchronization is necessary on the
>> part of the vendor code in the create/remove callbacks; does that sound
>> about right?
> v1/v2 did that; v3/v4 hold the list lock only while the device is added
> to the mdev list. v4 also adds a note regarding locking to the
> documentation.

I'll add some synchronization to the read/update of available instances.

>