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 <freude@xxxxxxxxxxxxxxxxxx> 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.
ready.
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.
Regards,
Pierre
--
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany
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