linux-kernel - Re: [PATCH v7 01/15] s390/vfio-ap: store queue struct in hash table for quick access

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Date:   Tue, 28 Apr 2020 12:07:26 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     Harald Freudenberger <freude@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, borntraeger@...ibm.com, cohuck@...hat.com,
        mjrosato@...ux.ibm.com, pmorel@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        jjherne@...ux.ibm.com, fiuczy@...ux.ibm.com
Subject: Re: [PATCH v7 01/15] s390/vfio-ap: store queue struct in hash table
 for quick access

On Mon, 27 Apr 2020 17:48:58 -0400
Tony Krowiak <akrowiak@...ux.ibm.com> wrote:

> 
> 
> On 4/27/20 11:17 AM, Halil Pasic wrote:
> > On Mon, 27 Apr 2020 15:05:23 +0200
> > Harald Freudenberger <freude@...ux.ibm.com> wrote:
> >
> >> On 24.04.20 05:57, Halil Pasic wrote:
> >>> On Tue,  7 Apr 2020 15:20:01 -0400
> >>> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
> >>>   
> >>>> Rather than looping over potentially 65535 objects, let's store the
> >>>> structures for caching information about queue devices bound to the
> >>>> vfio_ap device driver in a hash table keyed by APQN.
> >>> @Harald:
> >>> Would it make sense to make the efficient lookup of an apqueue base
> >>> on its APQN core AP functionality instead of each driver figuring it out
> >>> on it's own?
> >>>
> >>> If I'm not wrong the zcrypt device/driver(s) must the problem of
> >>> looking up a queue based on its APQN as well.
> >>>
> >>> For instance struct ep11_cprb has a target_id filed
> >>> (arch/s390/include/uapi/asm/zcrypt.h).
> >>>
> >>> Regards,
> >>> Halil
> >> Hi Halil
> >>
> >> no, the zcrypt drivers don't have this problem. They build up their own device object which
> >> includes a pointer to the base ap device.
> > I'm a bit confused. Doesn't your code loop first trough the ap_card
> > objects to find the APID portion of the APQN, and then loop the queue
> > list of the matching card to find the right ap_queue object? Or did I
> > miss something? Isn't that what _zcrypt_send_ep11_cprb() does? Can you
> > point me to the code that avoids the lookup (by apqn) for zcrypt?
> 
> The code you reference, _zcrypt_send_ep11_cprb(), does loop through
> each queue associated with each card, but it doesn't appear to be 
> looking for
> a queue with a particular APQN. It appears to be looking for a queue
> meeting a specific set of conditions. At least that's my take after 
> taking a very
> brief look at the code, so I'm not sure that applies here.
> 

One of the possible conditions is that the APQN is in the targets array.
Please have another look at the code below, is_desired_ep11_queue()
and is_desired_ep11_card() do APQI and APID part of the check
respectively:

        for_each_zcrypt_card(zc) {
                /* Check for online EP11 cards */
                if (!zc->online || !(zc->card->functions & 0x04000000))
                        continue;
                /* Check for user selected EP11 card */
                if (targets &&
                    !is_desired_ep11_card(zc->card->id, target_num, targets))
                        continue;
                /* check if device node has admission for this card */
                if (!zcrypt_check_card(perms, zc->card->id))
                        continue;
                /* get weight index of the card device  */
                weight = speed_idx_ep11(func_code) * zc->speed_rating[SECKEY];
                if (zcrypt_card_compare(zc, pref_zc, weight, pref_weight))
                        continue;
                for_each_zcrypt_queue(zq, zc) {
                        /* check if device is online and eligible */
                        if (!zq->online ||
                            !zq->ops->send_ep11_cprb ||
                            (targets &&
                             !is_desired_ep11_queue(zq->queue->qid,
                                                    target_num, targets)))


Yes the size of targets may or may not be 1 (example for size == 1 is
the invocation form ep11_cryptsingle()) and the respective costs
depend on the usual size of the array. Since the goal of the whole
exercise seems to be to pick a single queue, and we settle with the first
suitable (first not in the input array, but in our lists) that is
suitable, I assumed we wouldn't need many hashtable lookups.

Regards,
Halil

> >
> >
> > If you look at the new function of vfio_ap_get_queue(unsigned long apqn)
> > it basically about finding the queue based on the apqn, with the
> > difference that it is vfio specific.
> >
> > Regards,
> > Halil
> >
> >> However, this is not a big issue, as the ap_bus holds a list of ap_card objects and within each
> >> ap_card object there exists a list of ap_queues.
> >
> >
> >
>