linux-kernel - Re: [PATCH v6 2/7] s390: ap: new vfio_ap

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Date:   Thu, 28 Mar 2019 13:53:25 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Harald Freudenberger <freude@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>, borntraeger@...ibm.com
Cc:     alex.williamson@...hat.com, cohuck@...hat.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, frankja@...ux.ibm.com, pasic@...ux.ibm.com,
        david@...hat.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, mimu@...ux.ibm.com
Subject: Re: [PATCH v6 2/7] s390: ap: new vfio_ap_queue structure

On 27/03/2019 12:00, Harald Freudenberger wrote:
> On 26.03.19 21:45, Tony Krowiak wrote:
>> On 3/22/19 10:43 AM, Pierre Morel wrote:
>>> The AP interruptions are assigned on a queue basis and

...snip...

>>> +static void vfio_ap_free_queue(int apqn, struct ap_matrix_mdev *matrix_mdev)
>>> +{
>>> +    struct vfio_ap_queue *q;
>>> +
>>> +    q = vfio_ap_get_queue(apqn, &matrix_mdev->qlist);
>>> +    if (!q)
>>> +        return;
>>> +    q->matrix_mdev = NULL;
>>> +    vfio_ap_mdev_reset_queue(q);
>>
>> I'm wondering if it's necessary to reset the queue here. The only time
>> a queue is used is when a guest using the mdev device is started. When
>> that guest is terminated, the fd for the mdev device is closed and the
>> mdev device's release callback is invoked. The release callback resets
>> the queues assigned to the mdev device. Is it really necessary to
>> reset the queue again when it is unassigned even if there would have
>> been no subsequent activity?
> When I understand this here right this code is called when a queue goes
> away from the guest but is still reserved for use by the vfio dd. So it is
> possible to assign the queue now to another guest. But then it makes
> sense to clear all the entries in the millicode queue because a pending
> reply could be "received" by the wrong guest.
> 
> If this function is just called on remove of a queue device where the
> device goes back to the AP bus, a reset is not needed.

You are right Harald, the function is called when un-assigning a queue 
from a mediated device, so it must be reset before to assign it to another.


Regards,
Pierre

-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany