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Subject: Re: [PATCH v7 04/15] s390/vfio-ap: implement in-use callback for
 vfio_ap driver
To:     Tony Krowiak <akrowiak@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, freude@linux.ibm.com, borntraeger@de.ibm.com,
        mjrosato@linux.ibm.com, pasic@linux.ibm.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com,
        jjherne@linux.ibm.com, fiuczy@linux.ibm.com
References: <20200407192015.19887-1-akrowiak@linux.ibm.com>
 <20200407192015.19887-5-akrowiak@linux.ibm.com>
 <20200416131845.3ef6b3b5.cohuck@redhat.com>
 <5cf7d611-e30c-226d-0d3d-d37170f117f4@linux.ibm.com>
From:   Pierre Morel <pmorel@linux.ibm.com>
Date:   Fri, 17 Apr 2020 13:23:03 +0200
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On 2020-04-16 16:45, Tony Krowiak wrote:
> 
> 
> On 4/16/20 7:18 AM, Cornelia Huck wrote:
>> On Tue,  7 Apr 2020 15:20:04 -0400
>> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>>
>>> Let's implement the callback to indicate when an APQN
>>> is in use by the vfio_ap device driver. The callback is
>>> invoked whenever a change to the apmask or aqmask would
>>> result in one or more queue devices being removed from the driver. The
>>> vfio_ap device driver will indicate a resource is in use
>>> if the APQN of any of the queue devices to be removed are assigned to
>>> any of the matrix mdevs under the driver's control.
>>>
>>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>>> ---
>>>   drivers/s390/crypto/vfio_ap_drv.c     |  1 +
>>>   drivers/s390/crypto/vfio_ap_ops.c     | 47 +++++++++++++++++----------
>>>   drivers/s390/crypto/vfio_ap_private.h |  2 ++
>>>   3 files changed, 33 insertions(+), 17 deletions(-)
>>> @@ -1369,3 +1371,14 @@ void vfio_ap_mdev_remove_queue(struct ap_queue 
>>> *queue)
>>>       kfree(q);
>>>       mutex_unlock(&matrix_dev->lock);
>>>   }
>>> +
>>> +bool vfio_ap_mdev_resource_in_use(unsigned long *apm, unsigned long 
>>> *aqm)
>>> +{
>>> +    bool in_use;
>>> +
>>> +    mutex_lock(&matrix_dev->lock);
>>> +    in_use = vfio_ap_mdev_verify_no_sharing(NULL, apm, aqm) ? true : 
>>> false;
>> Maybe
>>
>> in_use = !!vfio_ap_mdev_verify_no_sharing(NULL, apm, aqm);
>>
>> ?
> 
> To be honest, I find the !! expression very confusing. Every time I see 
> it, I have
> to spend time thinking about what the result of !! is going to be. I think
> the statement should be left as-is because it more clearly expresses
> the intent.


In other places you use
"
         ret = vfio_ap_mdev_verify_no_sharing(matrix_mdev);
         if (ret)
                 goto share_err;
"
then why use a boolean here?

If you want to return a boolean and you do not want to use !! you can do:

  ...
   ret = vfio_ap_mdev_verify_no_sharing(NULL, apm, aqm);
...
   return (ret) ? false : true;

> 
>>
>>> +    mutex_unlock(&matrix_dev->lock);
>>> +
>>> +    return in_use;
>>> +}
> 

-- 
Pierre Morel
IBM Lab Boeblingen