[PATCH v5 00/12] s390x: CPU Topology

Pierre Morel
2021-12-09T13:46:04Z

Hi,

The goal of this series is to implement CPU topology for S390, it
improves the preceeding series with the implementation of books and
drawers, of non uniform CPU topology and with documentation.

To use these patches, you will need the Linux series version 5.
You find it there:
https://lkml.org/lkml/2021/11/22/361

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 or follow the introduction here under.

A short introduction
====================

CPU Topology is described in the S390 POP with essentially the description
of two instructions:

PTF Perform Topology function used to poll for topology change
and used to set the polarization but this part is not part of this item.

STSI Store System Information and the SYSIB 15.1.x providing the Topology
configuration.

S390 Topology is a 6 levels hierarchical topology with up to 5 level
of containers. The last topology level, specifying the CPU cores.

This patch series only uses the two lower levels sockets and cores.

To get the information on the topology, S390 provides the STSI
instruction, which stores a structures providing the list of the
containers used in the Machine topology: the SYSIB.
A selector within the STSI instruction allow to chose how many topology
levels will be provide in the SYSIB.

Using the Topology List Entries (TLE) provided inside the SYSIB we
the Linux kernel is able to compute the information about the cache
distance between two cores and can use this information to take
scheduling decisions.

The design
==========

1) To be ready for hotplug, I chose an Object oriented design
of the topology containers:
- A node is a bridge on the SYSBUS and defines a "node bus"
- A drawer is hotplug on the "node bus"
- A book on the "drawer bus"
- A socket on the "book bus"
- And the CPU Topology List Entry (CPU-TLE)sits on the socket bus.
These objects will be enhanced with the cache information when
NUMA is implemented.

This also allows for easy retrieval when building the different SYSIB
for Store Topology System Information (STSI)

2) Perform Topology Function (PTF) instruction is made available to the
guest with a new KVM capability and intercepted in QEMU, allowing the
guest to pool for topology changes.

Features
========

- There is no direct match between IDs shown by:
- lscpu (unrelated numbered list),
- SYSIB 15.1.x (topology ID)

- The CPU number, left column of lscpu, is used to reference a CPU
by Linux tools
While the CPU address is used by QEMU for hotplug.

- Effect of -smp parsing on the topology with an example:
-smp 9,sockets=4,cores=4,maxcpus=16

We have 4 socket each holding 4 cores so that we have a maximum
of 16 CPU, 9 of them are active on boot. (Should be obvious)

# lscpu -e
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2d:L2i ONLINE CONFIGURED POLARIZATION ADDRESS
0 0 0 0 0 0 0:0:0:0 yes yes horizontal 0
1 0 0 0 0 1 1:1:1:1 yes yes horizontal 1
2 0 0 0 0 2 2:2:2:2 yes yes horizontal 2
3 0 0 0 0 3 3:3:3:3 yes yes horizontal 3
4 0 0 0 1 4 4:4:4:4 yes yes horizontal 4
5 0 0 0 1 5 5:5:5:5 yes yes horizontal 5
6 0 0 0 1 6 6:6:6:6 yes yes horizontal 6
7 0 0 0 1 7 7:7:7:7 yes yes horizontal 7
8 0 0 0 2 8 8:8:8:8 yes yes horizontal 8
#

- To plug a new CPU inside the topology one can simply use the CPU
address like in:

(qemu) device_add host-s390x-cpu,core-id=12
# lscpu -e
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2d:L2i ONLINE CONFIGURED POLARIZATION ADDRESS
0 0 0 0 0 0 0:0:0:0 yes yes horizontal 0
1 0 0 0 0 1 1:1:1:1 yes yes horizontal 1
2 0 0 0 0 2 2:2:2:2 yes yes horizontal 2
3 0 0 0 0 3 3:3:3:3 yes yes horizontal 3
4 0 0 0 1 4 4:4:4:4 yes yes horizontal 4
5 0 0 0 1 5 5:5:5:5 yes yes horizontal 5
6 0 0 0 1 6 6:6:6:6 yes yes horizontal 6
7 0 0 0 1 7 7:7:7:7 yes yes horizontal 7
8 0 0 0 2 8 8:8:8:8 yes yes horizontal 8
9 - - - - - ::: no yes horizontal 12
# chcpu -e 9
CPU 9 enabled
# lscpu -e
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2d:L2i ONLINE CONFIGURED POLARIZATION ADDRESS
0 0 0 0 0 0 0:0:0:0 yes yes horizontal 0
1 0 0 0 0 1 1:1:1:1 yes yes horizontal 1
2 0 0 0 0 2 2:2:2:2 yes yes horizontal 2
3 0 0 0 0 3 3:3:3:3 yes yes horizontal 3
4 0 0 0 1 4 4:4:4:4 yes yes horizontal 4
5 0 0 0 1 5 5:5:5:5 yes yes horizontal 5
6 0 0 0 1 6 6:6:6:6 yes yes horizontal 6
7 0 0 0 1 7 7:7:7:7 yes yes horizontal 7
8 0 0 0 2 8 8:8:8:8 yes yes horizontal 8
9 0 0 0 3 9 9:9:9:9 yes yes horizontal 12
#

It is up to the admin level, Libvirt for example, to pin the righ CPU to the right
vCPU, but as we can see without NUMA, chosing separate sockets for CPUs is not easy
without hotplug because without information the code will assign the vCPU and fill
the sockets one after the other.
Note that this is also the default behavior on the LPAR.

Conclusion
==========

This patch, together with the associated KVM patch allows to provide CPU topology
information to the guest.
Currently, only dedicated vCPU and CPU are supported and a NUMA topology can only
be handled using CPU hotplug inside the guest.

Regards,
Pierre

Pierre Morel (12):
s390x: cpu topology: update linux headers
s390x: SCLP: reporting the maximum nested topology entries
s390x: topology: CPU topology objects and structures
s390x: topology: implementating Store Topology System Information
s390x: CPU topology: CPU topology migration
s390x: kvm: topology: interception of PTF instruction
s390: topology: Adding books to CPU topology
s390: topology: Adding books to STSI
s390: topology: Adding drawers to CPU topology
s390: topology: Adding drawers to STSI
s390x: topology: implementing numa for the s390x topology
s390: Topology: documentation

docs/system/s390x/numa-cpu-topology.rst | 273 +++++++++++
hw/core/machine-smp.c | 42 +-
hw/core/machine.c | 22 +
hw/s390x/cpu-topology.c | 599 ++++++++++++++++++++++++
hw/s390x/meson.build | 1 +
hw/s390x/s390-virtio-ccw.c | 124 ++++-
hw/s390x/sclp.c | 1 +
include/hw/boards.h | 8 +
include/hw/s390x/cpu-topology.h | 97 ++++
include/hw/s390x/s390-virtio-ccw.h | 6 +
include/hw/s390x/sclp.h | 4 +-
linux-headers/linux/kvm.h | 2 +
qapi/machine.json | 12 +-
softmmu/vl.c | 6 +
target/s390x/cpu.h | 50 ++
target/s390x/cpu_features_def.h.inc | 1 +
target/s390x/cpu_models.c | 2 +
target/s390x/cpu_topology.c | 169 +++++++
target/s390x/gen-features.c | 3 +
target/s390x/kvm/kvm.c | 26 +
target/s390x/machine.c | 48 ++
target/s390x/meson.build | 1 +
22 files changed, 1481 insertions(+), 16 deletions(-)
create mode 100644 docs/system/s390x/numa-cpu-topology.rst
create mode 100644 hw/s390x/cpu-topology.c
create mode 100644 include/hw/s390x/cpu-topology.h
create mode 100644 target/s390x/cpu_topology.c

--
2.27.0

Changelog:

- since v4

- Added drawer and books to topology

- Added numa topology

- Added documentation

- since v3

- Added migration
(Thomas)

- Separated STSI instruction from KVM to prepare TCG
(Thomas)

- Take care of endianess to prepare TCG
(Thomas)

- Added comments on STSI CPU container and PFT instruction
(Thomas)

- Moved enabling the instructions as the last patch
(Thomas)