[v17,00/12] s390x: CPU Topology

Message

Pierre Morel March 9, 2023, 12:14 p.m. UTC

Hi,

What is new in this new spin:

A) shadow_entitlement stores the vertical entitlement
   It serves two purposes:

1) when the host specifies the entitlement, he may not be aware
   of the polarization and may provide entitlement which is not
   correct for the current polarization.
   For exemple, specifying a vertical_high/low/medium entitlement
   when defining a vCPU on the QEMU command line or hot plugging a vCPU.
   Then we keep in shadow_entitlement the vertical entitlement specified
   when the machine is in an horizontal polarization.
   (impact on patch ...interception-of-PTF... make me remove Thomas R-B)

2) When the guest specifies a polarization change this allows QEMU to
   remember the setting specified on vCPU creation.
   The use of set-cpu-topology qmp command is not necessary set the
   vertical entitlement after receiving the CPU_TOPOLOGY_CHANGE event
   but is only needed to modify it.

B) qapi/machine-common.json is created for definitions common to both
   qapi/machine.json and qapi/machine-target.json.
   @CpuS390Entitlement is defined there.

C) Drawers and books are added in the output of hmp_hotpluggable_cpus

D) Documentation: put QAPI interface into devel directory,
   spelling/rewriting corrections.


Implementation discussions
==========================

CPU models
----------

Since the facility 11, S390_FEAT_CONFIGURATION_TOPOLOGY is already
in the CPU model for old QEMU we could not activate it as usual from
KVM but needed a KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
Checking and enabling this capability enables facility 11,
S390_FEAT_CONFIGURATION_TOPOLOGY.

It is the responsibility of the admin to ensure the same CPU
model for source and target host in a migration.

Migration
---------

When the target guest is started, the Multi-processor Topology Change
Report (MTCR) bit is set during the creation of the vCPU by KVM.
We do not need to migrate its state, in the worst case, the target
guest will see the MTCR and actualize its view of the topology
without necessity, but this will be done only one time.

Reset
-----

Reseting the topology is done during subsystem reset, the
polarization is reset to horizontal polarization.

Topology attributes
-------------------

The topology attributes are carried by the CPU object and defined
on object creation.
In the case the new attributes, socket, book, drawer, dedicated,
polarity are not provided QEMU provides defaults values.

- Geometry defaults
  The geometry default are based on the core-id of the core to 
  fill the geometry in a monotone way starting with drawer 0,
  book 0, and filling socket 0 with the number of cores per socket,
  then filling socket 1, socket 2 ... etc until the book is complete
  and all books until the first drawer is complete before starting with
  the next drawer.

  This allows to keep existing start scripts and Libvirt existing
  interface until it is extended.

- Modifiers defaults
  Default polarization is horizontal
  Default dedication is not dedicated.

Dynamic topology modification
-----------------------------

QAPI interface is extended with:
- a command: 'x-set-cpu-topology'
- a query: extension of qmp 'query-cpus-fast'
- a query: extension of hmp 'hotpluggable-cpus'
- an event: 'CPU_POLARITY_CHANGE'

The admin may use query-cpus-fast to verify the topology provided
to the guest and x-set-cpu-topology to modify it.

The event CPU_POLARITY_CHANGE is sent when the guest successfuly 
uses the PTF(2) instruction to request a polarization change.
In that case, the admin is supposed to modify the CPU provisioning
accordingly.

Testing
=======

To use the QEMU patches, you will need Linux V6-rc1 or newer,
or use the following Linux mainline patches:

f5ecfee94493 2022-07-20 KVM: s390: resetting the Topology-Change-Report    
24fe0195bc19 2022-07-20 KVM: s390: guest support for topology function     
0130337ec45b 2022-07-20 KVM: s390: Cleanup ipte lock access and SIIF fac.. 

Currently this code is for KVM only, I have no idea if it is interesting
to provide a TCG patch. If ever it will be done in another series.


Documentation
=============

To have a better understanding of the S390x CPU Topology and its
implementation in QEMU you can have a look at the documentation in the
last patch of this series.

The admin will want to match the host and the guest topology, taking
into account that the guest does not recognize multithreading.
Consequently, two vCPU assigned to threads of the same real CPU should
preferably be assigned to the same socket of the guest machine.


Regards,
Pierre
Pierre Morel (12):
  s390x/cpu topology: add s390 specifics to CPU topology
  s390x/cpu topology: add topology entries on CPU hotplug
  target/s390x/cpu topology: handle STSI(15) and build the SYSIB
  s390x/sclp: reporting the maximum nested topology entries
  s390x/cpu topology: resetting the Topology-Change-Report
  s390x/cpu topology: interception of PTF instruction
  target/s390x/cpu topology: activate CPU topology
  qapi/s390x/cpu topology: set-cpu-topology qmp command
  machine: adding s390 topology to query-cpu-fast
  machine: adding s390 topology to info hotpluggable-cpus
  qapi/s390x/cpu topology: CPU_POLARIZATION_CHANGE qapi event
  docs/s390x/cpu topology: document s390x cpu topology

 docs/devel/index-internals.rst     |   1 +
 docs/devel/s390-cpu-topology.rst   | 146 ++++++++
 docs/system/s390x/cpu-topology.rst | 238 +++++++++++++
 docs/system/target-s390x.rst       |   1 +
 qapi/machine-common.json           |  22 ++
 qapi/machine-target.json           |  82 +++++
 qapi/machine.json                  |  26 +-
 include/hw/boards.h                |  10 +-
 include/hw/s390x/cpu-topology.h    |  82 +++++
 include/hw/s390x/s390-virtio-ccw.h |   6 +
 include/hw/s390x/sclp.h            |   4 +-
 target/s390x/cpu.h                 |  79 +++++
 target/s390x/kvm/kvm_s390x.h       |   1 +
 hw/core/machine-hmp-cmds.c         |   6 +
 hw/core/machine-qmp-cmds.c         |   2 +
 hw/core/machine-smp.c              |  48 ++-
 hw/core/machine.c                  |   4 +
 hw/s390x/cpu-topology.c            | 524 +++++++++++++++++++++++++++++
 hw/s390x/s390-virtio-ccw.c         |  27 +-
 hw/s390x/sclp.c                    |   5 +
 softmmu/vl.c                       |   6 +
 target/s390x/cpu-sysemu.c          |  13 +
 target/s390x/cpu.c                 |   7 +
 target/s390x/cpu_models.c          |   1 +
 target/s390x/kvm/cpu_topology.c    | 310 +++++++++++++++++
 target/s390x/kvm/kvm.c             |  42 ++-
 hw/s390x/meson.build               |   1 +
 qapi/meson.build                   |   1 +
 qemu-options.hx                    |   7 +-
 target/s390x/kvm/meson.build       |   3 +-
 30 files changed, 1686 insertions(+), 19 deletions(-)
 create mode 100644 docs/devel/s390-cpu-topology.rst
 create mode 100644 docs/system/s390x/cpu-topology.rst
 create mode 100644 qapi/machine-common.json
 create mode 100644 include/hw/s390x/cpu-topology.h
 create mode 100644 hw/s390x/cpu-topology.c
 create mode 100644 target/s390x/kvm/cpu_topology.c