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

[Pierre Morel]

On 30/04/2019 16:09, Pierre Morel wrote:
> On 30/04/2019 15:26, Halil Pasic wrote:
> > On Fri, 26 Apr 2019 15:01:27 +0200
> > Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:
> >
> > > +/**
> > > + * vfio_ap_clrirq: Disable Interruption for a APQN
> > > + *
> > > + * @dev: the device associated with the ap_queue
> > > + * @q:ÂÂ 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)
> > > +{
> > > +ÂÂÂ struct ap_qirq_ctrl aqic_gisa = {};
> > > +ÂÂÂ struct ap_queue_status status;
> > > +ÂÂÂ int checks = 10;
> > > +
> > > +ÂÂÂ status = ap_aqic(q->apqn, aqic_gisa, NULL);
> > > +ÂÂÂ if (!status.response_code) {
> > > +ÂÂÂÂÂÂÂ while (status.irq_enabled && checks--) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂ 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.
> >
> > > +ÂÂÂÂÂÂÂÂÂÂÂ status = ap_tapq(q->apqn, NULL);
> > > +ÂÂÂÂÂÂÂ }
> > > +ÂÂÂÂÂÂÂ if (checks >= 0)
> > > +ÂÂÂÂÂÂÂÂÂÂÂ 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.

My answer is not clear: ap_aqic() is used only once, during the bus 
probe, in the all code to enable interrupt and is never used to disable 
interrupt.

Interrupt disabling is only done by using ap_zapq() or ap_rapq() which 
can not be intercepted.


> Yes, thanks, I will keep it as is, may be just play with msleep()time 
> and retry count.
>
> Regards,
> Pierre
>
> > Regards,
> > Halil
> >
> > > +ÂÂÂÂÂÂÂ else
> > > +ÂÂÂÂÂÂÂÂÂÂÂ WARN_ONCE("%s: failed disabling IRQ", __func__);
> > > +ÂÂÂ }
> > > +
> > > +ÂÂÂ return status;
> > > +}


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