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Re: [Qemu-devel] [PATCH v3 6/7] s390x/kvm: handle AP instruction interce

From: Tony Krowiak
Subject: Re: [Qemu-devel] [PATCH v3 6/7] s390x/kvm: handle AP instruction interception
Date: Thu, 5 Apr 2018 12:38:04 -0400
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On 04/03/2018 05:36 AM, Cornelia Huck wrote:
On Mon, 2 Apr 2018 12:36:27 -0400
Tony Krowiak <address@hidden> wrote:

On 03/26/2018 05:03 AM, Pierre Morel wrote:
On 26/03/2018 10:32, David Hildenbrand wrote:
On 16.03.2018 00:24, Tony Krowiak wrote:
+    /*
+     * The Query Configuration Information (QCI) function (fc == 4)
does not
+     * set a response code in reg 1, so check for that along with the
+     * AP feature.
+     */
+    if ((fc != 4) && s390_has_feat(S390_FEAT_AP)) {
+        env->regs[1] = 0x10000;
+        return 0;
+    }
This would imply an operation exception in case fc==4, which sounds very
It depends but I think that the S390_FEAT_AP_QUERY_CONFIG_INFO must be
to know what to answer.
If the feature is there, QCI must be answered correctly.
This is an interesting proposition which raises several issues that will
need to
be addressed. The only time the PQAP(QCI) instruction is intercepted is
* A vfio-ap device is NOT defined for the guest because the vfio_ap
device driver
    will set ECA.28 and the PQAP(QCI) instruction will be interpreted
* STFLE.12 is set for the guest

You say that the QCI must be answered correctly, but what is the correct
If we return the matrix - i.e., APM, ADM and AQM - configured via the
matrix device's sysfs attributes files, then if any APQNs are defined in
the matrix,
we will have to handle *ALL* AP instructions by executing them on behalf
of the
guest. I suppose we could return an empty matrix in which case the AP
bus will come
up without any devices on the guest, but what is the expectation of an
admin who
deliberately configures the mediated matrix device? Should we forego
handling interception
of AP instructions and consider a guest configuration that turns on
S390_FEAT_AP but
does not define a vfio-ap device to be erroneous and terminate starting
of the guest?
Anybody have any thoughts?
Hard to really give good advice without access to the documentation, but:
- If we tell the guest that the feature is available, but it does not
   get any cards to use, returning an empty matrix makes the most sense
   to me.
- I would not tie starting the guest to the presence of a vfio-ap
   device. Having the feature available in theory but without any
   devices actually being usable by the guest does not really sound
   wrong (can we hotplug this later?)
For this phase of development, it is my opinion that introducing AP instruction
interception handlers is superfluous for the following reasons:

1. Interception handling was introduced solely to ensure an operation exception would not be injected into the guest when CPU model feature for AP (i.e., ap=on) is specified but a VFIO AP device (i.e., -device vfio-ap,sysfsdev=$path)
   is not.

2. The implementation of guest dedicated crypto adapters uses AP instruction
   interpretation to virtualize AP devices for a guest. As such, the NQAP,
   DQAP and most variants of the PQAP instructions will not be

3. Hot plugging AP devices is not being supported for this phase of development.

It is my opinion that introducing these interception handlers at this time is
unnecessary and risks opening a can of worms that would be
better dealt with in a subsequent phase. For that reason and the reasons stated
above, I think the better option is to terminate starting the guest if the
CPU model feature for AP is enabled but an AP device is not defined for the
guest. This restriction, of course, will be removed when hot plug is implemented
in a subsequent development phase.

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