Re: [PATCH v7 09/22] s390: vfio-ap: register matrix device with VFIO mdev framework

Re: [PATCH v7 09/22] s390: vfio-ap: register matrix device with VFIO mdev framework

[Halil Pasic]

On 07/30/2018 01:28 PM, Cornelia Huck wrote:
> On Mon, 30 Jul 2018 13:24:24 +0200
> Pierre Morel <pmorel@linux.ibm.com> wrote:
> 
>> On 30/07/2018 11:49, Cornelia Huck wrote:
>>> On Fri, 27 Jul 2018 16:17:47 +0200
>>> Halil Pasic <pasic@linux.ibm.com> wrote:
>>>   
>>>> On 07/26/2018 09:54 PM, Christian Borntraeger wrote:
>>>>> @@ -65,6 +66,21 @@ static int vfio_ap_matrix_dev_create(void)
>>>>>     {
>>>>>     	int ret;
>>>>>     
>>>>> +	mutex_init(&matrix_dev.lock);
>>>>> +	INIT_LIST_HEAD(&matrix_dev.mdev_list);
>>>>> +
>>>>> +	/* Test if PQAP(QCI) instruction is available */
>>>>> +	if (test_facility(12)) {
>>>>> +		ret = ap_qci(&matrix_dev.info);
>>>>> +		if (ret && (ret != -EOPNOTSUPP)) {
>>>> After Connie's curiosity was piqued I gave this another look. If
>>>> I read the ap_qci() documentation and code correctly, it can return
>>>> either 0 or -EOPNOTSUPP, but nothing else. So basically this
>>>> is a dead branch.
>>> Can it return -EOPNOTSUPP if facility 12 is present?
>>
>> I do not think it is reasonable to continue if
>> we stated that facility 12 is present but the ap_qci function failed.
>>
>> So I propose that we return an error and break the insmod.
> 
> So, would that be a bug? If yes, returning an error makes sense.
> 
> (I can't check the documentation to find out about the interaction
> between facility 12 and PQAP(QCI)...)

If we are in z architectural mode and  facility 12 is indicated and
ap_qci() returns an error code, it's because of an exception while
executing the PQAP instruction. The reason can be a bad input pointer
or other stuff. It is likely a bug. I'm not sure how this interacts
with machine checks though. I think we should report the problem
and give up.  

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Re: [PATCH v7 09/22] s390: vfio-ap: register matrix device with VFIO mdev framework

[Cornelia Huck]

On Wed, 1 Aug 2018 17:34:26 -0400
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> On 07/27/2018 07:26 AM, Halil Pasic wrote:
> >
> >
> > On 07/27/2018 01:17 PM, Cornelia Huck wrote:  
> >> On Fri, 27 Jul 2018 13:13:40 +0200
> >> Halil Pasic <pasic@linux.ibm.com> wrote:
> >>  
> >>>>> @@ -65,6 +66,21 @@ static int vfio_ap_matrix_dev_create(void)
> >>>>>    {
> >>>>>        int ret;
> >>>>>    +    mutex_init(&matrix_dev.lock);
> >>>>> +    INIT_LIST_HEAD(&matrix_dev.mdev_list);
> >>>>> +
> >>>>> +    /* Test if PQAP(QCI) instruction is available */
> >>>>> +    if (test_facility(12)) {
> >>>>> +        ret = ap_qci(&matrix_dev.info);
> >>>>> +        if (ret && (ret != -EOPNOTSUPP)) {  
> >>>>
> >>>> So, is facility 12 a pre-req for PQAP(QCI)? Can the code work if
> >>>> matrix_dev.info is not populated?  
> >>>
> >>> Yes and yes. It's basically only used in vfio_ap_matrix_init() and it
> >>> has fallbacks.  
> >>
> >> Ok. So facility 12 is required but not sufficient for PQAP(QCI) to be
> >> available? (Just asking as I cannot check the doc; if it works with
> >> fallbacks I'm fine.)  
> 
> I took a look at the ap_qci() function. It returns:
> 
>      0:           success
>      -EOPNOTSUPP: returned when an exception occurs executing the
>                   PQAP(QCI) instruction
> 
> I conclude that if STFLE.12 is set, then the PQAP(QCI) instruction
> should work. If -ENOPNOTSUPP is returned, then this would be a
> serious error and I think we should return an error from this function.

Sounds reasonable to me.

> 
> >>  
> >
> > AFAIR it should be sufficient in our case. My guess is that the non 0
> > return code stands for program exceptions that are not related to the
> > availability of QCI.  
> 
> 
> 
>