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

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Message-Id: <18532145-abeb-1251-926e-edbc6fa0bcb0@linux.ibm.com>
Date:   Tue, 3 Jul 2018 14:20:11 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Harald Freudenberger <freude@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        kwankhede@...dia.com, bjsdjshi@...ux.vnet.ibm.com,
        pbonzini@...hat.com, alex.williamson@...hat.com,
        pmorel@...ux.vnet.ibm.com, alifm@...ux.vnet.ibm.com,
        mjrosato@...ux.vnet.ibm.com, jjherne@...ux.vnet.ibm.com,
        thuth@...hat.com, pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com,
        Tony Krowiak <akrowiak@...ux.ibm.com>
Subject: Re: [PATCH v6 21/21] s390: doc: detailed specifications for AP
 virtualization



On 07/03/2018 01:52 PM, Cornelia Huck wrote:
> On Tue, 3 Jul 2018 11:22:10 +0200
> Halil Pasic <pasic@...ux.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?

>> 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.

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.
> 
> 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