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>

docs/devel/s390-cpu-topology.rst

@@ -0,0 +1,170 @@
+QAPI interface for S390 CPU topology
+====================================
+
+The following sections will explain the QAPI interface for S390 CPU topology
+with the help of exemplary output.
+For this, let's assume that QEMU has been started with the following
+command, defining 4 CPUs, where CPU[0] is defined by the -smp argument and will
+have default values:
+
+.. 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,entitlement=high \
+    -device z14-s390x-cpu,core-id=11,entitlement=low \
+    -device z14-s390x-cpu,core-id=112,entitlement=high \
+   ...
+
+Additions to query-cpus-fast
+----------------------------
+
+The command query-cpus-fast allows querying 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"
+    }
+  ]
+ }
+
+
+QAPI command: set-cpu-topology
+------------------------------
+
+The command set-cpu-topology allows modifying 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 mandatory parameter and every
+unspecified parameter keeps its previous value.
+
+QAPI event CPU_POLARIZATION_CHANGE
+----------------------------------
+
+When a guest 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 entity administrating QEMU 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, giving 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
+
+  {
+    "timestamp": {
+      "seconds": 1687870305,
+      "microseconds": 566299
+    },
+    "event": "CPU_POLARIZATION_CHANGE",
+    "data": {
+      "polarization": "horizontal"
+    }
+  }
+
+QAPI query command: query-s390x-cpu-polarization
+------------------------------------------------
+
+The query command query-s390x-cpu-polarization returns the current
+CPU polarization of the machine.
+In this case the guest previously issued a PTF(1) to request vertical polarization:
+
+.. code-block:: QMP
+
+    { "execute": "query-s390x-cpu-polarization" }
+    {
+        "return": {
+          "polarization": "vertical"
+        }
+    }