Re: [PATCH v7 3/4] s390: ap: implement PAPQ AQIC interception in kernel

Pierre Morel
2019-04-30T14:10:10Z

On 30/04/2019 15:26, Halil Pasic wrote:
> On Fri, 26 Apr 2019 15:01:27 +0200
> Pierre Morel <pmorel@linux.ibm.com> wrote:
>
>> +/**
>> + * vfio_ap_clrirq: Disable Interruption for a APQN
>> + *
>> + * @dev: the device associated with the ap_queue
>> + * @q: the vfio_ap_queue holding AQIC parameters
>> + *
>> + * Issue the host side PQAP/AQIC
>> + * On success: unpin the NIB saved in *q and unregister from GIB
>> + * interface
>> + *
>> + * Return the ap_queue_status returned by the ap_aqic()
>> + */
>> +static struct ap_queue_status vfio_ap_clrirq(struct vfio_ap_queue *q)
>> +{
>> + struct ap_qirq_ctrl aqic_gisa = {};
>> + struct ap_queue_status status;
>> + int checks = 10;
>> +
>> + status = ap_aqic(q->apqn, aqic_gisa, NULL);
>> + if (!status.response_code) {
>> + while (status.irq_enabled && checks--) {
>> + msleep(20);
>
> Hm, that seems like a lot of time to me. And I suppose we are holding the
> kvm lock: e.g. no other instruction can be interpreted by kvm in the
> meantime.
>
>> + status = ap_tapq(q->apqn, NULL);
>> + }
>> + if (checks >= 0)
>> + vfio_ap_free_irq_data(q);
>
> Actually we don't have to wait for the async part to do it's magic
> (indicated by the status.irq_enabled --> !status.irq_enabled transition)
> in the instruction handler. We have to wait so we can unpin the NIB but
> that could be done async (e.g. workqueue).
>
> BTW do you have any measurements here? How many msleep(20) do we
> experience for one clear on average?

No idea but it is probably linked to the queue state and usage history.
I can use a lower sleep time and increment the retry count.

>
> If linux is not using clear (you told so offline, and I also remember
> something similar), we can probably get away with something like this,
> and do it properly (from performance standpoint) later.

In the Linux AP code it is only used once, in the explicit
ap_queue_enable_interruption() function.

Yes, thanks, I will keep it as is, may be just play with msleep()time
and retry count.

Regards,
Pierre

>
> Regards,
> Halil
>
>> + else
>> + WARN_ONCE("%s: failed disabling IRQ", __func__);
>> + }
>> +
>> + return status;
>> +}
>

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