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

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 3 Jul 2018 15:58:37 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On 07/03/2018 03:25 PM, 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.
> >   
> 
> I'm still confused. If it's about frequency of access (as hinted
> by block devices accessed more or less permanently) I'm not sure
> there is a substantial difference. I guess there are scenarios where
> the AP domain is used very seldom (e.g. protected keys --> most of
> the crypto ops done by CPACF but AP unwraps at the beginning), but
> there are such scenarios for block too.
> 
> If it's about (persistent) state, I guess it again depends on the
> scenario and on the type of the card. But I may be wrong.

So, let's turn this around: Why do you think that dasd (and not qeth or
whatever) is a good model for ap device unbinding? Because I really
fail to get it... maybe the ap driver maintainers can chime in.

> 
> >>  
> >>>> 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 was thinking about both directions. Re-classifying a device form
> pass-through to normal should also be possible. But the document only
> talks about one direction.

Presumably because it (rightfully) focuses on setting up vfio-ap?

> 
> I'm not familiar with the existing host drivers. If we can say 'Hey,
> unbind is perfectly safe at any time: no per-cautions need to be considered!'
> I'm very happy with that. Although I would find it a bit surprising.
> 
> I just wanted to make sure this is not something we forget.
> 
> >>
> >> 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?
> >   
> 
> My other email scratches this topic. AFAIK we don't have a solution for
> that yet. Nor we have a good understanding of how and to what extent
> is statically given what is given. E.g. if one wants to re-partition my AP
> resources (and at some point one will have to at least do the initial
> re-partitioning) do I need a reboot for the changes to take effect? Or
> is this 'known' variable during the uptime of an OS.

I think that is really out of scope for this file, which I'd expect to
explain how vfio-ap basically works and which incantations I need to
give crypto devices to a guest. It should NOT focus on administrative
tasks; this should either be delegated to the likes of libvirt or
documented in a "how to use crypto cards with kvm" kind of technical
writeup. If there's a limitation (e.g. you can't easily unbind again),
write a line here.