[PATCH v16 11/11] docs/s390x/cpu topology: document s390x cpu topology

Pierre Morel
2023-02-22T14:21:42Z

Add some basic examples for the definition of cpu topology
in s390x.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
---
docs/system/s390x/cpu-topology.rst | 378 +++++++++++++++++++++++++++++
docs/system/target-s390x.rst | 1 +
2 files changed, 379 insertions(+)
create mode 100644 docs/system/s390x/cpu-topology.rst

diff --git a/docs/system/s390x/cpu-topology.rst b/docs/system/s390x/cpu-topology.rst
new file mode 100644
index 0000000000..d470e28b97
--- /dev/null
+++ b/docs/system/s390x/cpu-topology.rst
@@ -0,0 +1,378 @@
+CPU topology on s390x
+=====================
+
+Since QEMU 8.0, CPU topology on s390x provides up to 3 levels of
+topology containers: drawers, books, sockets, defining a tree shaped
+hierarchy.
+
+The socket container contains one or more CPU entries consisting
+of a bitmap of three dentical CPU attributes:
+
+- CPU type
+- polarization entitlement
+- dedication
+
+Note also that since 7.2 threads are no longer supported in the topology
+and the ``-smp`` command line argument accepts only ``threads=1``.
+
+Prerequisites
+-------------
+
+To use CPU topology a Linux QEMU/KVM machine providing the CPU topology facility
+(STFLE bit 11) is required.
+
+However, since this facility has been enabled by default in an early version
+of QEMU, we use a capability, ``KVM_CAP_S390_CPU_TOPOLOGY``, to notify KVM
+that QEMU supports CPU topology.
+
+Enabling CPU topology
+---------------------
+
+Currently, CPU topology is only enabled in the host model by default.
+
+Enabling CPU topology in a CPU model is done by setting the CPU flag
+``ctop`` to ``on`` like in:
+
+.. code-block:: bash
+
+ -cpu gen16b,ctop=on
+
+Having the topology disabled by default allows migration between
+old and new QEMU without adding new flags.
+
+Default topology usage
+----------------------
+
+The CPU topology can be specified on the QEMU command line
+with the ``-smp`` or the ``-device`` QEMU command arguments.
+
+If none of the containers attributes (drawers, books, sockets) are
+specified for the ``-smp`` flag, the number of these containers
+is ``1`` .
+
+.. code-block:: bash
+
+ -smp cpus=5,drawer=1,books=1,sockets=8,cores=4,maxcpus=32
+
+or
+
+.. code-block:: bash
+
+ -smp cpus=5,sockets=8,cores=4,maxcpus=32
+
+When a CPU is defined by the ``-smp`` command argument, its position
+inside the topology is calculated by adding the CPUs to the topology
+based on the core-id starting with core-0 at position 0 of socket-0,
+book-0, drawer-0 and filling all CPUs of socket-0 before to fill socket-1
+of book-0 and so on up to the last socket of the last book of the last
+drawer.
+
+When a CPU is defined by the ``-device`` command argument, the
+tree topology attributes must be all defined or all not defined.
+
+.. code-block:: bash
+
+ -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1
+
+or
+
+.. code-block:: bash
+
+ -device gen16b-s390x-cpu,core-id=1,dedication=true
+
+If none of the tree attributes (drawer, book, sockets), are specified
+for the ``-device`` argument, as for all CPUs defined on the ``-smp``
+command argument the topology tree attributes will be set by simply
+adding the CPUs to the topology based on the core-id starting with
+core-0 at position 0 of socket-0, book-0, drawer-0.
+
+QEMU will not try to solve collisions and will report an error if the
+CPU topology, explicitely or implicitely defined on a ``-device``
+argument collides with the definition of a CPU implicitely defined
+on the ``-smp`` argument.
+
+When the topology modifier attributes are not defined for the
+``-device`` command argument they takes following default values:
+
+- dedication: ``false``
+- entitlement: ``medium``
+
+
+Hot plug
+++++++++
+
+New CPUs can be plugged using the device_add hmp command as in:
+
+.. code-block:: bash
+
+ (qemu) device_add gen16b-s390x-cpu,core-id=9
+
+The same placement of the CPU is derived from the core-id as described above.
+
+The topology can of course be fully defined:
+
+.. code-block:: bash
+
+ (qemu) device_add gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1
+
+
+Examples
+++++++++
+
+In the following machine we define 8 sockets with 4 cores each.
+
+.. code-block:: bash
+
+ $ qemu-system-s390x -m 2G \
+ -cpu gen16b,ctop=on \
+ -smp cpus=5,sockets=8,cores=4,maxcpus=32 \
+ -device host-s390x-cpu,core-id=14 \
+
+A new CPUs can be plugged using the device_add hmp command as before:
+
+.. code-block:: bash
+
+ (qemu) device_add gen16b-s390x-cpu,core-id=9
+
+The core-id defines the placement of the core in the topology by
+starting with core 0 in socket 0 up to maxcpus.
+
+In the example above:
+
+* There are 5 CPUs provided to the guest with the ``-smp`` command line
+ They will take the core-ids 0,1,2,3,4
+ As we have 4 cores in a socket, we have 4 CPUs provided
+ to the guest in socket 0, with core-ids 0,1,2,3.
+ The last cpu, with core-id 4, will be on socket 1.
+
+* the core with ID 14 provided by the ``-device`` command line will
+ be placed in socket 3, with core-id 14
+
+* the core with ID 9 provided by the ``device_add`` qmp command will
+ be placed in socket 2, with core-id 9
+
+
+Polarization, entitlement and dedication
+----------------------------------------
+
+Polarization
+++++++++++++
+
+The polarization is an indication given by the ``guest`` to the host
+that it is able to make use of CPU provisioning information.
+The guest indicates the polarization by using the PTF instruction.
+
+Polarization is define two models of CPU provisioning: horizontal
+and vertical.
+
+The horizontal polarization is the default model on boot and after
+subsystem reset in which the guest considers all vCPUs being having
+an equal provisioning of CPUs by the host.
+
+In the vertical polarization model the guest can make use of the
+vCPU entitlement information provided by the host to optimize
+kernel thread scheduling.
+
+A subsystem reset puts all vCPU of the configuration into the
+horizontal polarization.
+
+Entitlement
++++++++++++
+
+The vertical polarization specifies that guest's vCPU can get
+different real CPU provisions:
+
+- a vCPU with vertical high entitlement specifies that this
+ vCPU gets 100% of the real CPU provisioning.
+
+- a vCPU with vertical medium entitlement specifies that this
+ vCPU shares the real CPU with other vCPUs.
+
+- a vCPU with vertical low entitlement specifies that this
+ vCPU only gets real CPU provisioning when no other vCPUs needs it.
+
+In the case a vCPU with vertical high entitlement does not use
+the real CPU, the unused "slack" can be dispatched to other vCPU
+with medium or low entitlement.
+
+The admin specifies a vCPU as ``dedicated`` when the vCPU is fully dedicated
+to a single real CPU.
+
+The dedicated bit is an indication of affinity of a vCPU for a real CPU
+while the entitlement indicates the sharing or exclusivity of use.
+
+Defining the topology on command line
+-------------------------------------
+
+The topology can entirely be defined using -device cpu statements,
+with the exception of CPU 0 which must be defined with the -smp
+argument.
+
+For example, here we set the position of the cores 1,2,3 to
+drawer 1, book 1, socket 2 and cores 0,9 and 14 to drawer 0,
+book 0, socket 0 with all horizontal polarization and not dedicated.
+The core 4, will be set on its default position on socket 1
+(since we have 4 core per socket) and we define it with dedication and
+vertical high entitlement.
+
+.. code-block:: bash
+
+ $ qemu-system-s390x -m 2G \
+ -cpu gen16b,ctop=on \
+ -smp cpus=1,sockets=8,cores=4,maxcpus=32 \
+ \
+ -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1 \
+ -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=2 \
+ -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=3 \
+ \
+ -device gen16b-s390x-cpu,drawer-id=0,book-id=0,socket-id=0,core-id=9 \
+ -device gen16b-s390x-cpu,drawer-id=0,book-id=0,socket-id=0,core-id=14 \
+ \
+ -device gen16b-s390x-cpu,core-id=4,dedicated=on,polarization=3 \
+
+QAPI interface for topology
+---------------------------
+
+Let's start QEMU with the following command:
+
+.. code-block:: bash
+
+ qemu-system-s390x \
+ -enable-kvm \
+ -cpu z14,ctop=on \
+ -smp 1,drawers=3,books=3,sockets=2,cores=2,maxcpus=36 \
+ \
+ -device z14-s390x-cpu,core-id=19,polarization=3 \
+ -device z14-s390x-cpu,core-id=11,polarization=1 \
+ -device z14-s390x-cpu,core-id=112,polarization=3 \
+ ...
+
+and see the result when using the QAPI interface.
+
+addons to query-cpus-fast
++++++++++++++++++++++++++
+
+The command query-cpus-fast allows the admin to query the topology
+tree and modifiers for all configured vCPUs.
+
+.. code-block:: QMP
+
+ { "execute": "query-cpus-fast" }
+ {
+ "return": [
+ {
+ "dedicated": false,
+ "thread-id": 536993,
+ "props": {
+ "core-id": 0,
+ "socket-id": 0,
+ "drawer-id": 0,
+ "book-id": 0
+ },
+ "cpu-state": "operating",
+ "entitlement": "medium",
+ "qom-path": "/machine/unattached/device[0]",
+ "cpu-index": 0,
+ "target": "s390x"
+ },
+ {
+ "dedicated": false,
+ "thread-id": 537003,
+ "props": {
+ "core-id": 19,
+ "socket-id": 1,
+ "drawer-id": 0,
+ "book-id": 2
+ },
+ "cpu-state": "operating",
+ "entitlement": "high",
+ "qom-path": "/machine/peripheral-anon/device[0]",
+ "cpu-index": 19,
+ "target": "s390x"
+ },
+ {
+ "dedicated": false,
+ "thread-id": 537004,
+ "props": {
+ "core-id": 11,
+ "socket-id": 1,
+ "drawer-id": 0,
+ "book-id": 1
+ },
+ "cpu-state": "operating",
+ "entitlement": "low",
+ "qom-path": "/machine/peripheral-anon/device[1]",
+ "cpu-index": 11,
+ "target": "s390x"
+ },
+ {
+ "dedicated": true,
+ "thread-id": 537005,
+ "props": {
+ "core-id": 112,
+ "socket-id": 0,
+ "drawer-id": 3,
+ "book-id": 2
+ },
+ "cpu-state": "operating",
+ "entitlement": "high",
+ "qom-path": "/machine/peripheral-anon/device[2]",
+ "cpu-index": 112,
+ "target": "s390x"
+ }
+ ]
+ }
+
+
+set-cpu-topology
+++++++++++++++++
+
+The command set-cpu-topology allows the admin to modify the topology
+tree or the topology modifiers of a vCPU in the configuration.
+
+.. code-block:: QMP
+
+ -> { "execute": "set-cpu-topology",
+ "arguments": {
+ "core-id": 11,
+ "socket-id": 0,
+ "book-id": 0,
+ "drawer-id": 0,
+ "entitlement": low,
+ "dedicated": false
+ }
+ }
+ <- {"return": {}}
+
+The core-id parameter is the only non optional parameter and every
+unspecified parameter keeps its previous value.
+
+event CPU_POLARIZATION_CHANGE
++++++++++++++++++++++++++++++
+
+When a guest is requests a modification of the polarization,
+QEMU sends a CPU_POLARIZATION_CHANGE event.
+
+When requesting the change, the guest only specifies horizontal or
+vertical polarization.
+It is the job of the admin to set the dedication and fine grained vertical entitlement
+in response to this event.
+
+Note that a vertical polarized dedicated vCPU can only have a high
+entitlement, this gives 6 possibilities for vCPU polarization:
+
+- Horizontal
+- Horizontal dedicated
+- Vertical low
+- Vertical medium
+- Vertical high
+- Vertical high dedicated
+
+Example of the event received when the guest issues the CPU instruction
+Perform Topology Function PTF(0) to request an horizontal polarization:
+
+.. code-block:: QMP
+
+ <- { "event": "CPU_POLARIZATION_CHANGE",
+ "data": { "polarization": 0 },
+ "timestamp": { "seconds": 1401385907, "microseconds": 422329 } }
diff --git a/docs/system/target-s390x.rst b/docs/system/target-s390x.rst
index f6f11433c7..94c981e732 100644
--- a/docs/system/target-s390x.rst
+++ b/docs/system/target-s390x.rst
@@ -34,3 +34,4 @@ Architectural features
.. toctree::
s390x/bootdevices
s390x/protvirt
+ s390x/cpu-topology
--
2.31.1