linux-kernel - Re: [PATCH v11 01/14] s390/vfio-ap: No need to disable IRQ after queue reset

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Date:   Mon, 2 Nov 2020 09:35:48 -0500
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ux.ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, mjrosato@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        fiuczy@...ux.ibm.com, frankja@...ux.ibm.com, david@...hat.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com
Subject: Re: [PATCH v11 01/14] s390/vfio-ap: No need to disable IRQ after
 queue reset



On 10/30/20 11:43 PM, Halil Pasic wrote:
> On Fri, 30 Oct 2020 16:37:04 -0400
> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
>
>> On 10/30/20 1:42 PM, Halil Pasic wrote:
>>> On Thu, 29 Oct 2020 19:29:35 -0400
>>> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
>>>   
>>>>>> @@ -1177,7 +1166,10 @@ static int vfio_ap_mdev_reset_queues(struct mdev_device *mdev)
>>>>>>     			 */
>>>>>>     			if (ret)
>>>>>>     				rc = ret;
>>>>>> -			vfio_ap_irq_disable_apqn(AP_MKQID(apid, apqi));
>>>>>> +			q = vfio_ap_get_queue(matrix_mdev,
>>>>>> +					      AP_MKQID(apid, apqi));
>>>>>> +			if (q)
>>>>>> +				vfio_ap_free_aqic_resources(q);
>>>>> Is it safe to do vfio_ap_free_aqic_resources() at this point? I don't
>>>>> think so. I mean does the current code (and vfio_ap_mdev_reset_queue()
>>>>> in particular guarantee that the reset is actually done when we arrive
>>>>> here)? BTW, I think we have a similar problem with the current code as
>>>>> well.
>>>> If the return code from the vfio_ap_mdev_reset_queue() function
>>>> is zero, then yes, we are guaranteed the reset was done and the
>>>> queue is empty.
>>> I've read up on this and I disagree. We should discuss this offline.
>> Maybe you are confusing things here; my statement is specific to the return
>> code from the vfio_ap_mdev_reset_queue() function, not the response code
>> from the PQAP(ZAPQ) instruction. The vfio_ap_mdev_reset_queue()
>> function issues the PQAP(ZAPQ) instruction and if the status response code
>> is 0 indicating the reset was successfully initiated, it waits for the
>> queue to empty. When the queue is empty, it returns 0 to indicate
>> the queue is reset.
>> If the queue does not become empty after a period of
>> time,
>> it will issue a warning (WARN_ON_ONCE) and return 0. In that case, I suppose
>> there is no guarantee the reset was done, so maybe a change needs to be
>> made there such as a non-zero return code.
>>
> I've overlooked the wait for empty. Maybe that return 0 had a part in
> it. I now remember me insisting on having the wait code added when the
> interrupt support was in the make. Sorry!
>
> If we have given up on out of retries retries, we are in trouble anyway.
>   
>>>   
>>>>     The function returns a non-zero return code if
>>>> the reset fails or the queue the reset did not complete within a given
>>>> amount of time, so maybe we shouldn't free AQIC resources when
>>>> we get a non-zero return code from the reset function?
>>>>   
>>> If the queue is gone, or broken, it won't produce interrupts or poke the
>>> notifier bit, and we should clean up the AQIC resources.
>> True, which is what the code provided by this patch does; however,
>> the AQIC resources should be cleaned up only if the KVM pointer is
>> not NULL for reasons discussed elsewhere.
> Yes, but these should be cleaned up before the KVM pointer becomes
> null. We don't want to keep the page with the notifier byte pinned
> forever, or?

No, we do not want to keep the page forever. I probably should
have been clearer. There are times we do a reset - e.g., on remove
of the mdev - at which time there should be no KVM pointer, or
else the remove will not be allowed. Of course, we won't do the
reset either, so I guess you can ignore my comment. If there is
no KVM pointer yet a page remains pinned, something bad
happened.