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

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Message-ID: <d1aae333-3f10-bd21-257f-baf8f8df7a24@linux.ibm.com>
Date:   Fri, 30 Oct 2020 16:53:04 -0400
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 1:54 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);
> [..]
>
>>> Under what circumstances do we expect !q? If we don't, then we need to
>>> complain one way or another.
>> In the current code (i.e., prior to introducing the subsequent hot
>> plug patches), an APQN can not be assigned to an mdev unless it
>> references a queue device bound to the vfio_ap device driver; however,
>> there is nothing preventing a queue device from getting unbound
>> while the guest is running (one of the problems mostly resolved by this
>> series). In that case, q would be NULL.
> But if the queue does not belong to us any more it does not make sense
> call vfio_ap_mdev_reset_queue() on it's APQN, or?

This is precisely why we prevent a queue from being taken away
from vfio_ap (the in-use callback) when its APQN is assigned to an
mdev in this patch series. On the other hand, this is a very good
point.

>
> I think we should have
>
> if(!q)
> 	continue;
> at the very beginning of the loop body, or we want to be sure that q is
> not null.

I agree, I'll go ahead and make this change.




>   
>