[PATCH v9 10/10] docs/s390x: document s390x cpu topology

[Pierre Morel <pmorel@linux.ibm.com>]

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 | 88 ++++++++++++++++++++++++++++++
 1 file changed, 88 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..00977d4319
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+++ b/docs/system/s390x/cpu_topology.rst
@@ -0,0 +1,88 @@
+CPU Topology on s390x
+=====================
+
+CPU Topology on S390x provides up to 4 levels of topology containers:
+drawers, books, sockets and CPUs.
+While the three higher level containers, Containers Topology List Entries,
+(Containers TLE) define a tree hierarchy, the lowest level of topology
+definition, the CPU Topology List Entry (CPU TLE), provides the placement
+of the CPUs inside the last container.
+
+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,
+the capability ``KVM_CAP_S390_CPU_TOPOLOGY`` is needed to indicate to KVM
+that QEMU support CPU Topology.
+
+Indicating the CPU topology to the Virtual Machine
+--------------------------------------------------
+
+The CPU Topology, number of drawers, number of books per drawers, number of
+sockets per book and number of cores per sockets is specified with the
+``-smp`` qemu command arguments.
+
+Like in :
+
+.. code-block:: sh
+    -smp cpus=1,drawers=3,books=4,sockets=2,cores=8,maxcpus=192
+
+If drawers or books are not specified, their default to 1.
+
+New CPUs can be plugged using the device_add hmp command like in:
+
+.. code-block:: sh
+   (qemu) device_add host-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, book 0 and drawer 0 up to the maximum
+core number of the last socket of the last book in the last drawer.
+
+In the example above:
+
+* the core with ID 9 will be placed in container (0,0,1), as core 9
+  of CPU TLE 0 of socket 1 in book 0 from drawer 0.
+* the core ID 0 is defined by the -smp cpus=1 command and will be
+  placed as core 0 in CPU TLE 0 of container (0,0,0)
+
+Note that the core ID is machine wide and the CPU TLE masks provided
+by the STSI instruction will be:
+
+* in socket 0: 0x80000000 (core id 0)
+* in socket 1: 0x00400000 (core id 9)
+
+Indicating the CPU topology to the Guest
+----------------------------------------
+
+The guest can query for topology changes using the PTF instruction.
+In case of a topology change it can request the new topology by issuing
+STSI instructions specifying the level of detail required, drawer with
+STSI(15.1.4) or books STSI(15.1.3).
+
+The virtual machine will fill the provided buffer with the count of
+drawers (MAG4), books per drawer (MAG3), sockets per book (MAG2) and
+cores per socket (MAG1).
+
+Note that the STSI(15.1.2) is special in two ways:
+
+* When the firmware detect a change in the values calculated for STSI(15.1.2)
+  it will trigger the report of the topology change for the PTF instruction.
+
+Migration
+---------
+
+For virtio-ccw machines older than s390-virtio-ccw-7.2, CPU Topoogy is
+by default disabled.
+
+CPU Topoogy is by default enabled for s390-virtio-ccw-7.2 and newer machines.
+
+Enabling the CPU topology on older Machine is done by setting the global
+option ''topology-disable'' to false before enabling cpu topology with the
+cpu feature "ctop" like in:
+
+.. code-block:: sh
+   -machine s390-ccw-virtio-3.0,accel=kvm,topology-disable=false
+   -cpu z14,ctop=on
-- 
2.31.1