Re: [PATCH v6 21/21] s390: doc: detailed specifications for AP virtualization

[Tony Krowiak <akrowiak@linux.ibm.com>]

On 07/03/2018 09:25 AM, Cornelia Huck wrote:
> On Tue, 3 Jul 2018 14:20:11 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
>
>> On 07/03/2018 01:52 PM, Cornelia Huck wrote:
>>> On Tue, 3 Jul 2018 11:22:10 +0200
>>> Halil Pasic <pasic@linux.ibm.com> wrote:
>>>    
>> [..]
>>>> Let me try to invoke the DASD analogy. If one for some reason wants to detach
>>>> a DASD the procedure to follow seems to be (see
>>>> https://www.ibm.com/support/knowledgecenter/en/linuxonibm/com.ibm.linux.z.lgdd/lgdd_t_dasd_online.html)
>>>> the following:
>>>> 1) Unmount.
>>>> 2) Offline possibly using safe_offline.
>>>> 3) Detach.
>>>>
>>>> Detaching a disk that is currently doing I/O asks for trouble, so the admin is encouraged
>>>> to make sure there is no pending I/O.
>>> I don't think we can use dasd (block devices) as a good analogy for
>>> every kind of device (for starters, consider network devices).
>>>    
>> I did not use it for every kind of device. I used it for AP. I'm
>> under the impression you find the analogy inappropriate. If, could
>> you please explain why?
> I don't think block devices (which are designed to be more or less
> permanently accessed, e.g. by mounting a file system) have the same
> semantics as ap devices (which exist as a backend for crypto requests).
> Not everything that makes sense for a block device makes sense for
> other devices as well, and I don't think it makes sense here.
>
>>>> In case of AP you can interpret my 'in use' as the queue is not empty. In my understanding
>>>> unbind is supposed to be hard (I used the word radical). That's why I compared it to pulling
>>>> a cable. So that's why I ask is there stuff the admin is supposed to do before doing the
>>>> unbind.
>>> Are you asking for a kind of 'quiescing' operation? I would hope that
>>> the crypto drivers already can deal with that via flushing the queue,
>>> not allowing new requests, or whatever. This is not the block device
>>> case.
>>>    
>> The current implementation of vfio-ap which is a crypto driver too certainly
>> can not deal 'with that'. Whether the rest of the drivers can, I don't
>> know. Maybe Tony can tell.
> If the current implementation of vfio-ap cannot deal with it (by
> cleaning up, blocking, etc.), it needs at the very least be documented
> so that it can be implemented later. I do not know what the SIE will or
> won't do to assist here (e.g., if you're removing it from some masks,
> the device will already be inaccessible to the guest). But the part you
> were referring to was talking about the existing host driver anyway,
> wasn't it?

I addressed this in the cover letter and included a comment in the remove
callback for the vfio_ap driver. The goal is to provide this in the next
patch series.

>
>> I'm aware of the fact that AP adapters are not block devices. But
>> as stated above I don't understand what is the big difference regarding
>> the unbind operation.
>>
>>> Anyway, this is an administrative issue. If you don't have a clear
>>> concept which devices are for host usage and which for guest usage, you
>>> already have problems.
>> I'm trying to understand the whole solution. I agree, this is an administrative
>> issue. But the document is trying to address such administrative issues.
> I'd assume "know which devices are for the host and which devices are
> for the guests" to be a given, no?
>
>>> Speaking of administrative issues, is there libvirt support for vfio-ap
>>> under development? It would be helpful to validate the approach.
>> I full-heartedly agree. I guess Tony will have to answer this one too.
>>
>> Regards,
>> Halil
>>