system/s390x/cpu-topology.rst

1 .. _cpu-topology-s390x:
3 CPU topology on s390x
6 Since QEMU 8.2, CPU topology on s390x provides up to 3 levels of
8 tree-shaped hierarchy.
13 - CPU type
14 - entitlement
15 - dedication
17 Each bit set in the bitmap correspond to a core-id of a vCPU with matching
20 This documentation provides general information on S390 CPU topology,
22 For information on how to modify the S390 CPU topology and how to
23 monitor polarization changes, see ``docs/devel/s390-cpu-topology.rst``.
26 -------------
31 The s390x host needs to use a Linux kernel v6.0 or newer (which provides the so-called
33 CPU topology facility via the so-called STFLE bit 11 to the VM).
36 ---------------------
38 Currently, CPU topology is enabled by default only in the "host" CPU model.
41 ``ctop`` to ``on`` as in:
43 .. code-block:: bash
45    -cpu gen16b,ctop=on
47 Having the topology disabled by default allows migration between
50 Default topology usage
51 ----------------------
53 The CPU topology can be specified on the QEMU command line
54 with the ``-smp`` or the ``-device`` QEMU command arguments.
57 and the ``-smp`` command line argument accepts only ``threads=1``.
60 specified for the ``-smp`` flag, the number of these containers
65 .. code-block:: bash
67     -smp cpus=5,drawer=1,books=1,sockets=8,cores=4,maxcpus=32
71 .. code-block:: bash
73     -smp cpus=5,sockets=8,cores=4,maxcpus=32
75 When a CPU is defined by the ``-smp`` command argument, its position
77 based on the core-id starting with core-0 at position 0 of socket-0,
78 book-0, drawer-0 and filling all CPUs of socket-0 before filling socket-1
79 of book-0 and so on up to the last socket of the last book of the last
82 When a CPU is defined by the ``-device`` command argument, the
85 .. code-block:: bash
87     -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1
91 .. code-block:: bash
93     -device gen16b-s390x-cpu,core-id=1,dedicated=true
96 for the ``-device`` argument, like for all CPUs defined with the ``-smp``
98 adding the CPUs to the topology based on the core-id.
101 CPU topology defined explicitly or implicitly on a ``-device``
103 on the ``-smp`` argument.
106 ``-device`` command argument they takes following default values:
108 - dedicated: ``false``
109 - entitlement: ``medium``
117 .. code-block:: bash
119   (qemu) device_add gen16b-s390x-cpu,core-id=9
121 The placement of the CPU is derived from the core-id as described above.
125 .. code-block:: bash
127     (qemu) device_add gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1
135 .. code-block:: bash
137   $ qemu-system-s390x -accel kvm -m 2G \
138     -cpu gen16b,ctop=on \
139     -smp cpus=5,sockets=8,cores=4,maxcpus=32 \
140     -device host-s390x-cpu,core-id=14 \
144 .. code-block:: bash
146   (qemu) device_add gen16b-s390x-cpu,core-id=9
148 The core-id defines the placement of the core in the topology by
153 * There are 5 CPUs provided to the guest with the ``-smp`` command line
154   They will take the core-ids 0,1,2,3,4
156   to the guest in socket 0, with core-ids 0,1,2,3.
157   The last CPU, with core-id 4, will be on socket 1.
159 * the core with ID 14 provided by the ``-device`` command line will
160   be placed in socket 3, with core-id 14
163   be placed in socket 2, with core-id 9
167 ----------------------------------------
179 The horizontal polarization is the default model on boot and after
197 - a vCPU with vertical high entitlement specifies that this
200 - a vCPU with vertical medium entitlement specifies that this
203 - a vCPU with vertical low entitlement specifies that this
216 Defining the topology on the command line
217 -----------------------------------------
219 The topology can entirely be defined using -device cpu statements,
220 with the exception of CPU 0 which must be defined with the -smp
226 Core 4 will be set on its default position on socket 1
230 .. code-block:: bash
232   $ qemu-system-s390x -accel kvm -m 2G \
233     -cpu gen16b,ctop=on \
234     -smp cpus=1,sockets=8,cores=4,maxcpus=32 \
236     -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1 \
237     -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=2 \
238     -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=3 \
240     -device gen16b-s390x-cpu,drawer-id=0,book-id=0,socket-id=0,core-id=9 \
241     -device gen16b-s390x-cpu,drawer-id=0,book-id=0,socket-id=0,core-id=14 \
243     -device gen16b-s390x-cpu,core-id=4,dedicated=on,entitlement=high