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

From: Tony Krowiak
Subject: Re: [qemu-s390x] [RFC 19/19] s390/facilities: enable AP facilities needed by guest
Date: Fri, 1 Dec 2017 20:30:19 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0

On 11/03/2017 04:47 AM, Christian Borntraeger wrote:

On 11/02/2017 07:49 PM, Tony Krowiak wrote:
On 11/02/2017 11:53 AM, Christian Borntraeger wrote:
On 11/02/2017 04:36 PM, Tony Krowiak wrote:
On 11/02/2017 08:08 AM, Christian Borntraeger wrote:
On 10/16/2017 11:25 AM, Martin Schwidefsky wrote:
On Fri, 13 Oct 2017 13:39:04 -0400
Tony Krowiak <address@hidden> wrote:

Sets up the following facilities bits to enable the specified AP
facilities for the guest VM:
      * STFLE.12: Enables the AP Query Configuration Information
                  facility. The AP bus running in the guest uses
                  the information returned from this instruction
                  to configure AP adapters and domains for the
                  guest machine.
      * STFLE.15: Indicates the AP facilities test is available.
                  The AP bus running in the guest uses the

Signed-off-by: Tony Krowiak <address@hidden>
   arch/s390/tools/gen_facilities.c |    2 ++
   1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/arch/s390/tools/gen_facilities.c b/arch/s390/tools/gen_facilities.c
index 70dd8f1..eeaa7db 100644
--- a/arch/s390/tools/gen_facilities.c
+++ b/arch/s390/tools/gen_facilities.c
@@ -74,8 +74,10 @@ struct facility_def {
               8,  /* enhanced-DAT 1 */
               9,  /* sense-running-status */
               10, /* conditional sske */
+            12, /* AP query configuration */
               13, /* ipte-range */
               14, /* nonquiescing key-setting */
+            15, /* AP special-command facility */
               73, /* transactional execution */
               75, /* access-exception-fetch/store indication */
               76, /* msa extension 3 */
With this all KVM guests will always have the AP instructions available, no?
In principles I like this approach, but it differs from the way z/VM does 
there the guest will get an exception if it tries to execute an AP instruction
if there are no AP devices assigned to the guest. I wonder if there is a reason
why z/VM does it the way it does.
A good question. For LPAR it seems that you have AP instructions even if you 
no crypto cards.

I don't believe these facilities control whether or not AP instructions will be 

to the guest.
This is actually handled by your patch2 enabling the ECA bit.
I think we must decide if we want to be able to disable these instructions
via the cpu model. If yes we must then couple the facilities with the 
The ECA.28 bit controls whether instructions are intercepted or interpreted - 
i.e., handled via hardware
virtualization. If set, as is done in patch2, then instructions will be 
interpreted. I don't see how
that affects enabling or disabling AP instructions, unless we don't set ECA.28, 
intercept every instruction
and program check. Am I missing something here?
If we do not set ECA.28 these instructions intercept and we (the hypervisor) 
can then
decide what to do. For example we can give an PIC01 operation exception (illegal
instruction) - thats what we do today.

Now: if we want to be able to migrate a guest from a new kernel back to an old 
there must be a way to disable the new behaviour so that the user can configure 
a guest
that does NOT have these 3 instructions. That means, I want to bind the ap 
to a cpu model feature, so that we only enable ECA.28 and the facility bits, if 
feature is enabled in the CPU model. Otherwise we have no control on what 
when the guest issues these instructions.

Imagine what happens if we not do this and you migrate from an identical hw 
with an
identical libvirt/qemu but from a new kernel to an old kernel:

The guest boots starts up on the new kernel
guest kernel: drivers/s390/crypto/ap_bus.c  ap_module_init -> 
checks if the instructions work. They do and now the guest driver assumes that 
instructions will continue to work.

Now the guest is migrated back to an old kernel
sooner or later the ap_scan_bus kthread will run to scan the bus (or some 
crypto operation
is started) and the instruction will be rejected with a PIC01. kernel oops.
There are several scenarios that have to be accounted for, such as:
* Migrating from a linux host where both the KVM/kernel and QEMU support AP matrix devices to a guest host where neither the KVM/kernel nor QEMU support AP matrix
* Migrating from a linux host where both the KVM/kernel and QEMU support AP matrix devices to a guest host where the KVM/kernel does not support AP matrix devices
  but QEMU does;
* Starting a guest on a linux host where QEMU supports AP matrix devices and
  the KVM/kernel does not;
* etc.

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.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.

* 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.

If the AP bus running on the guest is expected to mimic the behavior of an
AP bus running on the host, then I think we need a CPU model feature for each
facility. Otherwise, I think we can group them within a CPU model feature
that indicates AP matrix devices are supported. What say you?

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