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

From: Tony Krowiak
Date: Tue Jul 03 2018 - 11:17:41 EST

On 07/03/2018 08:20 AM, Halil Pasic wrote:

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

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

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.

As stated in the cover letter, unbinding a queue from the vfio-ap device
driver is akin to a hot unplug. Hot plug/unplug is one of the goals of
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.

This section of the document is intended to describe how to provision AP queues
for dedicated guest usage and to show the relationship between the various objects
involved. While it does administrative actions, it is not intended to be an
administrator's guide.

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.