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Re: [PATCH v16 00/11] s390x: CPU Topology

From: Thomas Huth
Subject: Re: [PATCH v16 00/11] s390x: CPU Topology
Date: Mon, 27 Feb 2023 15:00:19 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0

On 22/02/2023 15.20, Pierre Morel wrote:

No big changes here, some bug corrections and comments modifications
following Thomas and Nina comments and Daniel and Markus reommandations.

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,

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


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 'query-cpus-fast'

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


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.


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.


Pierre Morel (11):
   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: activating CPU topology
   qapi/s390x/cpu topology: set-cpu-topology monitor command
   machine: adding s390 topology to query-cpu-fast
   qapi/s390x/cpu topology: CPU_POLARIZATION_CHANGE qapi event
   docs/s390x/cpu topology: document s390x cpu topology

  docs/system/s390x/cpu-topology.rst | 378 ++++++++++++++++++++
  docs/system/target-s390x.rst       |   1 +
  qapi/machine-target.json           |  81 +++++
  qapi/machine.json                  |  37 +-
  include/hw/boards.h                |  10 +-
  include/hw/s390x/cpu-topology.h    |  78 +++++
  include/hw/s390x/s390-virtio-ccw.h |   6 +
  include/hw/s390x/sclp.h            |   4 +-
  include/monitor/hmp.h              |   1 +
  target/s390x/cpu.h                 |  78 +++++
  target/s390x/kvm/kvm_s390x.h       |   1 +
  hw/core/machine-qmp-cmds.c         |   2 +
  hw/core/machine-smp.c              |  48 ++-
  hw/core/machine.c                  |   4 +
  hw/s390x/cpu-topology.c            | 534 +++++++++++++++++++++++++++++
  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    | 312 +++++++++++++++++
  target/s390x/kvm/kvm.c             |  42 ++-
  hmp-commands.hx                    |  17 +
  hw/s390x/meson.build               |   1 +
  qemu-options.hx                    |   7 +-
  target/s390x/kvm/meson.build       |   3 +-
  27 files changed, 1685 insertions(+), 19 deletions(-)
  create mode 100644 docs/system/s390x/cpu-topology.rst
  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

Any chance that you could also add some qtests for checking that the topology works as expected? I.e. set some topology via the command line, then use QMP to check whether all CPUs got the right settings?


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