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

Tony Krowiak
2018-07-03T16:36:39Z

On 07/02/2018 07:10 PM, Halil Pasic wrote:
>
>
> On 06/29/2018 11:11 PM, Tony Krowiak wrote:
>> This patch provides documentation describing the AP architecture and
>> design concepts behind the virtualization of AP devices. It also
>> includes an example of how to configure AP devices for exclusive
>> use of KVM guests.
>>
>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>
> I don't like the design of external interfaces except for:
> * cpu model features, and
> * reset handling.
>
> In particular:
>
> 1) The architecture is such that authorizing access (via APM, AQM and
> ADM)
> to an AP queue that is currently not configured (e.g. the card not
> physically
> plugged, or just configured off). That seems to be a perfectly normal use
> case.
>
> Your assign operations however enforce that the resource is bound to your
> driver, and thus the existence of the resource in the host.
>
> It is clear: we need to avoid passing trough resources to guests that
> are not
> dedicated for this purpose (e.g. a queue utilized by zcrypt). But IMHO
> we need a different mechanism.

Interesting that you wait until v6 to bring this up. I agree, this is a
normal
use case, but there is currently no mechanism in the AP bus for drivers to
reserve devices that are not yet configured. There is proposed solution
in the
works, but until such time that is available the only choice is to disallow
assignment of AP queues to a guest that are not bound to the vfio_ap
device driver.

>
>
> 2) I see no benefit in deferring the exclusivity check to
> vfio_ap_mdev_open().
> The downside is however pretty obvious: management software is
> notified about
> a 'bad configuration' only at an attempted guest start-up. And your
> current QEMU
> patches are not very helpful in conveying this piece of information.

It only becomes a 'bad configuration' if the two guests are started
concurrently.
Is there value in being able to configure two mediated devices with the same
queue if the intent is to never run two guests using those mediated devices
simultaneously? If so, then the only time the exclusivity check can be done
is when the guest opens the mediated device. If not, then we can certainly
prevent multiple mediated devices from being assigned the same queue.

In my view, while a mediated device is used by a guest, it is not a
guest and
can be configured any way an administrator prefers. If we get concurrence
that doing an exclusivity check when an adapter or domain is assigned to
the mediated device, I'll make that change.

>
>
> I've talked with Boris, and AFAIR he said this is not acceptable to
> him (@Boris
> can you confirm).

Then I suggest Boris participate in the review and explain why.

>
>
> 3) We indicate the reason for failure due to a configuration problem
> (exclusivity
> or resource allocation) via pr_err() that is via kernel messages. I
> don't think
> this is very tooling/management software friendly, and I hope we don't
> expect admins
> to work with the sysfs interface long term. I mean the effects of the
> admin actions
> are not very persistent. Thus if the interface is a painful one, we
> are talking
> about potentially frequent pain.

We have multiple layers of software, each with its own logging
facilities. Figuring
out what went wrong when a guest fails to start is always a painful
process IMHO.
Typically, one has to view the log for each component in the stack to
figure out
what went wrong and often times, still can't figure it out. Of course,
we can help
out here by having QEMU put out a better message when this problem
occurs. But the
bottom line is, does the community think that allowing an administrator
to configure
multiple mediated devices with the same queues have value? In other
words, are
there potential use cases that would required this?

>
>
> 4) If I were to act out the role of the administrator, I would prefer
> to think of
> specifying or changing the access controls of a guest in respect to AP
> (that is
> setting the AP matrix) as a single atomic operation -- which either
> succeeds or fails.

I don't understand what you are describing here. How would this be done?
Are you
suggesting the admin somehow provides the masks en masse?

>
>
> The operation should succeed for any valid configuration, and fail for
> any invalid
> on.
>
> The current piecemeal approach seems even less fitting if we consider
> changing the
> access controls of a running guest. AFAIK changing access controls for
> a running
> guest is possible, and I don't see a reason why should we artificially
> prohibit this.

Setting and clearing bits in the APM/AQM/ADM of a guest's CRYCB is
certainly possible,
but there is a lot more to it than merely setting and clearing bits.
What you seem
to be describing here is hot plug/unplug which I stated in the cover
letter is
forthcoming. It is currently prohibited for good reason.

>
>
> I think the current sysfs interface for manipulating the matrix is
> good for
> manual playing around, but I would prefer having an interface that is
> better
> suited for programs (e.g. ioctl).

That wouldn't be a problem, but do we have a use case for it?

>
>
> Regards,
> Halil