qapi: machine.json: change docs regarding CPU topology

Clarify roles of different architectures.
Also change things a bit in anticipation of additional members being
added.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Message-ID: <20231016183925.2384704-2-nsg@linux.ibm.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
[thuth: Updated some comments according to suggestions from Markus]
Signed-off-by: Thomas Huth <thuth@redhat.com>

qapi/machine.json

@@ -71,8 +71,7 @@
 #
 # @thread-id: ID of the underlying host thread
 #
-# @props: properties describing to which node/socket/core/thread
+# @props: properties associated with a virtual CPU, e.g. the socket id
-#     virtual CPU belongs to, provided if supported by board
 #
 # @target: the QEMU system emulation target, which determines which
 #     additional fields will be listed (since 3.0)
@@ -899,28 +898,35 @@
 # should be passed by management with device_add command when a CPU is
 # being hotplugged.
 #
+# Which members are optional and which mandatory depends on the
+# architecture and board.
+#
+# The ids other than the node-id specify the position of the CPU
+# within the CPU topology (as defined by the machine property "smp",
+# thus see also type @SMPConfiguration)
+#
 # @node-id: NUMA node ID the CPU belongs to
 #
-# @socket-id: socket number within node/board the CPU belongs to
+# @socket-id: socket number within CPU topology the CPU belongs to
 #
-# @die-id: die number within socket the CPU belongs to (since 4.1)
+# @die-id: die number within the parent container the CPU belongs to
+#    (since 4.1)
 #
-# @cluster-id: cluster number within die the CPU belongs to (since
+# @cluster-id: cluster number within the parent container the CPU
-#     7.1)
+#     belongs to (since 7.1)
 #
-# @core-id: core number within cluster the CPU belongs to
+# @core-id: core number within the parent container the CPU
+#     belongs to
 #
-# @thread-id: thread number within core the CPU belongs to
+# @thread-id: thread number within the core the CPU  belongs to
 #
-# Note: currently there are 6 properties that could be present but
+# Note: management should be prepared to pass through additional
-#     management should be prepared to pass through other properties
+#     properties with device_add.
-#     with device_add command to allow for future interface extension.
-#     This also requires the filed names to be kept in sync with the
-#     properties passed to -device/device_add.
 #
 # Since: 2.7
 ##
 { 'struct': 'CpuInstanceProperties',
+  # Keep these in sync with the properties device_add accepts
   'data': { '*node-id': 'int',
             '*socket-id': 'int',
             '*die-id': 'int',
@@ -1478,22 +1484,33 @@
 # Schema for CPU topology configuration.  A missing value lets QEMU
 # figure out a suitable value based on the ones that are provided.
 #
+# The members other than @cpus and @maxcpus define a topology of
+# containers.
+#
+# The ordering from highest/coarsest to lowest/finest is:
+# @sockets, @dies, @clusters, @cores, @threads.
+#
+# Different architectures support different subsets of topology
+# containers.
+#
+# For example, s390x does not have clusters and dies, and the socket
+# is the parent container of cores.
+#
 # @cpus: number of virtual CPUs in the virtual machine
 #
-# @sockets: number of sockets in the CPU topology
-#
-# @dies: number of dies per socket in the CPU topology
-#
-# @clusters: number of clusters per die in the CPU topology (since
-#     7.0)
-#
-# @cores: number of cores per cluster in the CPU topology
-#
-# @threads: number of threads per core in the CPU topology
-#
 # @maxcpus: maximum number of hotpluggable virtual CPUs in the virtual
 #     machine
 #
+# @sockets: number of sockets in the CPU topology
+#
+# @dies: number of dies per parent container
+#
+# @clusters: number of clusters per parent container (since 7.0)
+#
+# @cores: number of cores per parent container
+#
+# @threads: number of threads per core
+#
 # Since: 6.1
 ##
 { 'struct': 'SMPConfiguration', 'data': {