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

Harald Freudenberger
2020-04-28T10:57:26Z

On 28.04.20 12:07, Halil Pasic wrote:
> On Mon, 27 Apr 2020 17:48:58 -0400
> Tony Krowiak <akrowiak@linux.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@linux.ibm.com> wrote:
>>>
>>>> On 24.04.20 05:57, Halil Pasic wrote:
>>>>> On Tue, 7 Apr 2020 15:20:01 -0400
>>>>> Tony Krowiak <akrowiak@linux.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
again, this is all code related to zcrypt card and queues and has nothing directly to do with ap queue and ap cards.
If you want to have a look how this works for ap devices, have a look into the scan routines for the ap bus in ap_bus.c
There you can find a bus_for_each_device() which would fit together with the right matching function for your needs.
And this is exactly what Tony implemented in the first shot. However, as written I can provide something like that
for you.
One note for the improvement via hash list with the argument about the max 65535 objects.
Think about a real big machine which has currently up to 30 crypto cards (z15 GA1.5) which when CEX7S are
plugged appear as 60 crypto adapters and have up to 85 domains each. When all these crypto resources
are assigned to one LPAR we end up in 60x85 = 5100 APQNs. Well, of course with a hash you can improve
the linear search through an array or list but can you measure the performance gain and then compare this
to the complexity.  ... just some thoughts about beautifying code ...
>>>
>>> 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.
>>>
>>>