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Re: [qemu-s390x] [RFC 19/19] s390/facilities: enable AP facilities neede

From: Pierre Morel
Subject: Re: [qemu-s390x] [RFC 19/19] s390/facilities: enable AP facilities needed by guest
Date: Wed, 6 Dec 2017 10:15:51 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 05/12/2017 16:01, Tony Krowiak wrote:
On 12/05/2017 09:04 AM, Cornelia Huck wrote:
On Tue, 5 Dec 2017 08:52:57 +0100
Harald Freudenberger <address@hidden> wrote:

On 12/02/2017 02:30 AM, Tony Krowiak wrote:
I agree with your suggestion that defining a new CPU model feature is probably the best way to resolve this issue. The question is, should we define a single feature indicating whether AP instructions are installed and set features bits for the guest based on whether or not they are set in the linux host, or should we define additional CPU model features for turning features bits on and off? I guess it boils down to what behavior is expected for the AP bus running on the linux guest. Here is a rundown of the facilities bits associated with AP
and how they affect the behavior of the AP bus:

* STFLE.12 indicates whether the AP query function is available. If this bit    is not set, then the AP bus scan will only test domains 0-15. For example,    if adapters 4, 5, and 6 and domains 12 and 71 (0x47) are installed, then AP
   queues 04.0047, 05.0047 and 06.0047 will not be made available.
STFLE 12 is the indication for Query AP Configuration Information (QCI) available.
* STFLE.15 indicates whether the AP facilities test function is available. If    this bit is not set, then the CEX4, CEX5 and CEX6 device drivers discovered    by the AP bus scan will not get bound to any AP device drivers. Since the    AP matrix model supports only CEX4 and greater, no devices will be bound
   to any driver for a guest.
This T-Bit extension to the TAPQ subfunction is a must have. When kvm only supports CEX4 and upper then this bit could also act as the indicator for AP instructions available. Of course if you want to implement pure virtual full simulated AP without any real AP hardware on the host this bit can't
be the indicator.
It would probably make sense to group these two together. Or is there
any advantage in supporting only a part of it?
After thinking about this a little more, I've come to the conclusion that
all of this might be moot for the following reasons:

* If STFLE.12 is not set for the linux host, then AP bus scan running on
  the host will not detect any domains with a domain number higher than 15,
   so no AP queues with a queue index higher than 15 will be available to
   bind to the vfio_ap_matrix driver. Consequently, no domain higher than
  15 can be assigned to any guest. In this case, the AP bus scan running on
   the guest will never detect a domain higher than 15, regardless of the
   setting of STFLE.12 for the guest.

* If STFLE.15 is not set for the linux host, then then there will be no
   CEX4, CEX5 or CEX6 queues available to bind to the vfio_ap_matrix
   driver, so no AP adapters or domains can be assigned to any KVM guest.

The bottom line is the STFLE bit settings for the linux host will control
what APs are available to the KVM guest. Since STFLE.15 controls whether
any CEX4,5 or 6 devices are even available, I think this bit can be
combined into the feature that indicates whether AP is available. As long
as AP instructions are available on the linux host, I'm not sure whether
STFLE.12 needs a feature at all.

We are implementing VFIO with SIE interpretation.

1) Providing more:
The simple way is to provide to the guest only features existing on the host. If we do provide features not existing on the host we need to be able to emulate them. Even it is possible, it could be done in a future enhancement, but AFAIK it is not the goal of the current development.

2) Providing less:
On the other hand we can mask to the guest some of the features provided by the host if we can intercept the scanning of the features.

What I understand from this is that we need all these features being separately toggled to be able to be compatible with an older system even if we have a 1:1 host:guest features match in a first version.

If several features where introduced together in a new architecture and are available on all systems issued from this architecture we can then gather them in a set. (But I will wonder why we have several features then)

* STFLE.65 indicates whether AP interrupts are available. If this bit is not    set, then the AP bus will use polling instead of using interrupt handlers
   to process AP events.
So, does this indicate "adapter interrupts for AP" only? If so, we
should keep this separate and only enable it when we have the gisa etc.
Yes, this indicates AP interrupts only. The plan is to enable this when
GISA is available and we can implement interrupt processing.

If we want to be able to work on system where STFLE.65 is not available, even if GISA is available I think it would be interesting to have a Matrix implementation with only polling.



Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany

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