docs/s390x/cpu topology: document s390x cpu topology

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

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Co-developed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Message-ID: <20231016183925.2384704-15-nsg@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

1 files changed, 244 insertions, 0 deletions

diff --git a/docs/system/s390x/cpu-topology.rst b/docs/system/s390x/cpu-topology.rst
new file mode 100644
index 0000000..5133fdc
--- /dev/null
+++ b/docs/system/s390x/cpu-topology.rst
@@ -0,0 +1,244 @@
+.. _cpu-topology-s390x:
+
+CPU topology on s390x
+=====================
+
+Since QEMU 8.2, CPU topology on s390x provides up to 3 levels of
+topology containers: drawers, books and sockets. They define a
+tree-shaped hierarchy.
+
+The socket container has one or more CPU entries.
+Each of these CPU entries consists of a bitmap and three CPU attributes:
+
+- CPU type
+- entitlement
+- dedication
+
+Each bit set in the bitmap correspond to a core-id of a vCPU with matching
+attributes.
+
+This documentation provides general information on S390 CPU topology,
+how to enable it and explains the new CPU attributes.
+For information on how to modify the S390 CPU topology and how to
+monitor polarization changes, see ``docs/devel/s390-cpu-topology.rst``.
+
+Prerequisites
+-------------
+
+To use the CPU topology, you need to run with KVM on a s390x host that
+uses the Linux kernel v6.0 or newer (which provide the so-called
+``KVM_CAP_S390_CPU_TOPOLOGY`` capability that allows QEMU to signal the
+CPU topology facility via the so-called STFLE bit 11 to the VM).
+
+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`` as 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.
+
+Note also that since 7.2 threads are no longer supported in the topology
+and the ``-smp`` command line argument accepts only ``threads=1``.
+
+If none of the containers attributes (drawers, books, sockets) are
+specified for the ``-smp`` flag, the number of these containers
+is 1.
+
+Thus the following two options will result in the same topology:
+
+.. code-block:: bash
+
+    -smp cpus=5,drawer=1,books=1,sockets=8,cores=4,maxcpus=32
+
+and
+
+.. 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 filling 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 all be 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,dedicated=true
+
+If none of the tree attributes (drawer, book, sockets), are specified
+for the ``-device`` argument, like for all CPUs defined with the ``-smp``
+command argument the topology tree attributes will be set by simply
+adding the CPUs to the topology based on the core-id.
+
+QEMU will not try to resolve collisions and will report an error if the
+CPU topology defined explicitly or implicitly on a ``-device``
+argument collides with the definition of a CPU implicitly defined
+on the ``-smp`` argument.
+
+When the topology modifier attributes are not defined for the
+``-device`` command argument they takes following default values:
+
+- dedicated: ``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 placement of the CPU is derived from the core-id as described above.
+
+The topology can of course also 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 affects how the CPUs of a shared host are utilized/distributed
+among guests.
+The guest determines the polarization by using the PTF instruction.
+
+Polarization defines two models of CPU provisioning: horizontal
+and vertical.
+
+The horizontal polarization is the default model on boot and after
+subsystem reset. When horizontal polarization is in effect all vCPUs should
+have about equal resource provisioning.
+
+In the vertical polarization model vCPUs are unequal, but overall more resources
+might be available.
+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 the guest's vCPU can get
+different real CPU provisioning:
+
+- 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.
+
+A vCPU can be "dedicated" in which case 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 the 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 without defining entitlement or dedication.
+Core 4 will be set on its default position on socket 1
+(since we have 4 core per socket) and we define it as dedicated and
+with 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,entitlement=high
+
+The entitlement defined for the CPU 4 will only be used after the guest
+successfully enables vertical polarization by using the PTF instruction.