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>
This commit is contained in:
Pierre Morel
Thomas Huth
parent
154893a784
commit
0d177cdd2b
@ -1716,6 +1716,8 @@ S: Supported
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F: include/hw/s390x/cpu-topology.h
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F: hw/s390x/cpu-topology.c
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F: target/s390x/kvm/stsi-topology.c
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F: docs/devel/s390-cpu-topology.rst
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F: docs/system/s390x/cpu-topology.rst
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X86 Machines
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------------
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@ -14,6 +14,7 @@ Details about QEMU's various subsystems including how to add features to them.
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migration
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multi-process
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reset
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s390-cpu-topology
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s390-dasd-ipl
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tracing
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vfio-migration
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170
docs/devel/s390-cpu-topology.rst
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170
docs/devel/s390-cpu-topology.rst
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@ -0,0 +1,170 @@
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QAPI interface for S390 CPU topology
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====================================
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The following sections will explain the QAPI interface for S390 CPU topology
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with the help of exemplary output.
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For this, let's assume that QEMU has been started with the following
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command, defining 4 CPUs, where CPU[0] is defined by the -smp argument and will
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have default values:
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.. code-block:: bash
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qemu-system-s390x \
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-enable-kvm \
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-cpu z14,ctop=on \
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-smp 1,drawers=3,books=3,sockets=2,cores=2,maxcpus=36 \
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-device z14-s390x-cpu,core-id=19,entitlement=high \
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-device z14-s390x-cpu,core-id=11,entitlement=low \
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-device z14-s390x-cpu,core-id=112,entitlement=high \
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...
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Additions to query-cpus-fast
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----------------------------
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The command query-cpus-fast allows querying the topology tree and
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modifiers for all configured vCPUs.
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.. code-block:: QMP
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{ "execute": "query-cpus-fast" }
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{
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"return": [
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{
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"dedicated": false,
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"thread-id": 536993,
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"props": {
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"core-id": 0,
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"socket-id": 0,
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"drawer-id": 0,
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"book-id": 0
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},
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"cpu-state": "operating",
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"entitlement": "medium",
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"qom-path": "/machine/unattached/device[0]",
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"cpu-index": 0,
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"target": "s390x"
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},
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{
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"dedicated": false,
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"thread-id": 537003,
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"props": {
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"core-id": 19,
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"socket-id": 1,
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"drawer-id": 0,
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"book-id": 2
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},
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"cpu-state": "operating",
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"entitlement": "high",
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"qom-path": "/machine/peripheral-anon/device[0]",
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"cpu-index": 19,
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"target": "s390x"
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},
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{
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"dedicated": false,
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"thread-id": 537004,
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"props": {
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"core-id": 11,
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"socket-id": 1,
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"drawer-id": 0,
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"book-id": 1
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},
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"cpu-state": "operating",
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"entitlement": "low",
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"qom-path": "/machine/peripheral-anon/device[1]",
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"cpu-index": 11,
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"target": "s390x"
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},
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{
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"dedicated": true,
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"thread-id": 537005,
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"props": {
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"core-id": 112,
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"socket-id": 0,
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"drawer-id": 3,
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"book-id": 2
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},
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"cpu-state": "operating",
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"entitlement": "high",
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"qom-path": "/machine/peripheral-anon/device[2]",
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"cpu-index": 112,
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"target": "s390x"
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}
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]
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}
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QAPI command: set-cpu-topology
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------------------------------
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The command set-cpu-topology allows modifying the topology tree
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or the topology modifiers of a vCPU in the configuration.
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.. code-block:: QMP
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{ "execute": "set-cpu-topology",
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"arguments": {
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"core-id": 11,
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"socket-id": 0,
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"book-id": 0,
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"drawer-id": 0,
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"entitlement": "low",
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"dedicated": false
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}
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}
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{"return": {}}
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The core-id parameter is the only mandatory parameter and every
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unspecified parameter keeps its previous value.
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QAPI event CPU_POLARIZATION_CHANGE
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----------------------------------
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When a guest requests a modification of the polarization,
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QEMU sends a CPU_POLARIZATION_CHANGE event.
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When requesting the change, the guest only specifies horizontal or
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vertical polarization.
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It is the job of the entity administrating QEMU to set the dedication and fine
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grained vertical entitlement in response to this event.
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Note that a vertical polarized dedicated vCPU can only have a high
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entitlement, giving 6 possibilities for vCPU polarization:
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- Horizontal
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- Horizontal dedicated
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- Vertical low
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- Vertical medium
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- Vertical high
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- Vertical high dedicated
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Example of the event received when the guest issues the CPU instruction
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Perform Topology Function PTF(0) to request an horizontal polarization:
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.. code-block:: QMP
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{
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"timestamp": {
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"seconds": 1687870305,
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"microseconds": 566299
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},
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"event": "CPU_POLARIZATION_CHANGE",
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"data": {
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"polarization": "horizontal"
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}
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}
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QAPI query command: query-s390x-cpu-polarization
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------------------------------------------------
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The query command query-s390x-cpu-polarization returns the current
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CPU polarization of the machine.
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In this case the guest previously issued a PTF(1) to request vertical polarization:
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.. code-block:: QMP
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{ "execute": "query-s390x-cpu-polarization" }
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{
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"return": {
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"polarization": "vertical"
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}
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}
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244
docs/system/s390x/cpu-topology.rst
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244
docs/system/s390x/cpu-topology.rst
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@ -0,0 +1,244 @@
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.. _cpu-topology-s390x:
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CPU topology on s390x
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=====================
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Since QEMU 8.2, CPU topology on s390x provides up to 3 levels of
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topology containers: drawers, books and sockets. They define a
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tree-shaped hierarchy.
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The socket container has one or more CPU entries.
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Each of these CPU entries consists of a bitmap and three CPU attributes:
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- CPU type
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- entitlement
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- dedication
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Each bit set in the bitmap correspond to a core-id of a vCPU with matching
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attributes.
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This documentation provides general information on S390 CPU topology,
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how to enable it and explains the new CPU attributes.
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For information on how to modify the S390 CPU topology and how to
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monitor polarization changes, see ``docs/devel/s390-cpu-topology.rst``.
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Prerequisites
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-------------
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To use the CPU topology, you need to run with KVM on a s390x host that
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uses the Linux kernel v6.0 or newer (which provide the so-called
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``KVM_CAP_S390_CPU_TOPOLOGY`` capability that allows QEMU to signal the
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CPU topology facility via the so-called STFLE bit 11 to the VM).
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Enabling CPU topology
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---------------------
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Currently, CPU topology is only enabled in the host model by default.
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Enabling CPU topology in a CPU model is done by setting the CPU flag
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``ctop`` to ``on`` as in:
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.. code-block:: bash
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-cpu gen16b,ctop=on
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Having the topology disabled by default allows migration between
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old and new QEMU without adding new flags.
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Default topology usage
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----------------------
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The CPU topology can be specified on the QEMU command line
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with the ``-smp`` or the ``-device`` QEMU command arguments.
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Note also that since 7.2 threads are no longer supported in the topology
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and the ``-smp`` command line argument accepts only ``threads=1``.
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If none of the containers attributes (drawers, books, sockets) are
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specified for the ``-smp`` flag, the number of these containers
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is 1.
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Thus the following two options will result in the same topology:
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.. code-block:: bash
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-smp cpus=5,drawer=1,books=1,sockets=8,cores=4,maxcpus=32
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and
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.. code-block:: bash
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-smp cpus=5,sockets=8,cores=4,maxcpus=32
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When a CPU is defined by the ``-smp`` command argument, its position
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inside the topology is calculated by adding the CPUs to the topology
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based on the core-id starting with core-0 at position 0 of socket-0,
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book-0, drawer-0 and filling all CPUs of socket-0 before filling socket-1
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of book-0 and so on up to the last socket of the last book of the last
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drawer.
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When a CPU is defined by the ``-device`` command argument, the
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tree topology attributes must all be defined or all not defined.
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.. code-block:: bash
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-device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1
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or
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.. code-block:: bash
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-device gen16b-s390x-cpu,core-id=1,dedicated=true
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If none of the tree attributes (drawer, book, sockets), are specified
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for the ``-device`` argument, like for all CPUs defined with the ``-smp``
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command argument the topology tree attributes will be set by simply
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adding the CPUs to the topology based on the core-id.
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QEMU will not try to resolve collisions and will report an error if the
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CPU topology defined explicitly or implicitly on a ``-device``
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argument collides with the definition of a CPU implicitly defined
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on the ``-smp`` argument.
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When the topology modifier attributes are not defined for the
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``-device`` command argument they takes following default values:
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- dedicated: ``false``
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- entitlement: ``medium``
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Hot plug
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++++++++
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New CPUs can be plugged using the device_add hmp command as in:
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.. code-block:: bash
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(qemu) device_add gen16b-s390x-cpu,core-id=9
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The placement of the CPU is derived from the core-id as described above.
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The topology can of course also be fully defined:
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.. code-block:: bash
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(qemu) device_add gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1
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Examples
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++++++++
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In the following machine we define 8 sockets with 4 cores each.
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.. code-block:: bash
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$ qemu-system-s390x -m 2G \
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-cpu gen16b,ctop=on \
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-smp cpus=5,sockets=8,cores=4,maxcpus=32 \
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-device host-s390x-cpu,core-id=14 \
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A new CPUs can be plugged using the device_add hmp command as before:
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.. code-block:: bash
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(qemu) device_add gen16b-s390x-cpu,core-id=9
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The core-id defines the placement of the core in the topology by
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starting with core 0 in socket 0 up to maxcpus.
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In the example above:
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* There are 5 CPUs provided to the guest with the ``-smp`` command line
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They will take the core-ids 0,1,2,3,4
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As we have 4 cores in a socket, we have 4 CPUs provided
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to the guest in socket 0, with core-ids 0,1,2,3.
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The last CPU, with core-id 4, will be on socket 1.
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* the core with ID 14 provided by the ``-device`` command line will
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be placed in socket 3, with core-id 14
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* the core with ID 9 provided by the ``device_add`` qmp command will
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be placed in socket 2, with core-id 9
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Polarization, entitlement and dedication
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----------------------------------------
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Polarization
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++++++++++++
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The polarization affects how the CPUs of a shared host are utilized/distributed
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among guests.
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The guest determines the polarization by using the PTF instruction.
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Polarization defines two models of CPU provisioning: horizontal
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and vertical.
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The horizontal polarization is the default model on boot and after
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subsystem reset. When horizontal polarization is in effect all vCPUs should
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have about equal resource provisioning.
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In the vertical polarization model vCPUs are unequal, but overall more resources
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might be available.
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The guest can make use of the vCPU entitlement information provided by the host
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to optimize kernel thread scheduling.
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A subsystem reset puts all vCPU of the configuration into the
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horizontal polarization.
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Entitlement
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+++++++++++
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The vertical polarization specifies that the guest's vCPU can get
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different real CPU provisioning:
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- a vCPU with vertical high entitlement specifies that this
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vCPU gets 100% of the real CPU provisioning.
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- a vCPU with vertical medium entitlement specifies that this
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vCPU shares the real CPU with other vCPUs.
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- a vCPU with vertical low entitlement specifies that this
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vCPU only gets real CPU provisioning when no other vCPUs needs it.
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In the case a vCPU with vertical high entitlement does not use
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the real CPU, the unused "slack" can be dispatched to other vCPU
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with medium or low entitlement.
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A vCPU can be "dedicated" in which case the vCPU is fully dedicated to a single
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real CPU.
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The dedicated bit is an indication of affinity of a vCPU for a real CPU
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while the entitlement indicates the sharing or exclusivity of use.
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Defining the topology on the command line
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-----------------------------------------
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The topology can entirely be defined using -device cpu statements,
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with the exception of CPU 0 which must be defined with the -smp
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argument.
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For example, here we set the position of the cores 1,2,3 to
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drawer 1, book 1, socket 2 and cores 0,9 and 14 to drawer 0,
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book 0, socket 0 without defining entitlement or dedication.
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Core 4 will be set on its default position on socket 1
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(since we have 4 core per socket) and we define it as dedicated and
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with vertical high entitlement.
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.. code-block:: bash
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$ qemu-system-s390x -m 2G \
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-cpu gen16b,ctop=on \
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-smp cpus=1,sockets=8,cores=4,maxcpus=32 \
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\
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-device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1 \
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-device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=2 \
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-device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=3 \
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\
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-device gen16b-s390x-cpu,drawer-id=0,book-id=0,socket-id=0,core-id=9 \
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-device gen16b-s390x-cpu,drawer-id=0,book-id=0,socket-id=0,core-id=14 \
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\
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-device gen16b-s390x-cpu,core-id=4,dedicated=on,entitlement=high
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The entitlement defined for the CPU 4 will only be used after the guest
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successfully enables vertical polarization by using the PTF instruction.
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@ -34,3 +34,4 @@ Architectural features
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.. toctree::
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s390x/bootdevices
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s390x/protvirt
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s390x/cpu-topology
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@ -909,6 +909,8 @@
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# Which members are optional and which mandatory depends on the
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# architecture and board.
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#
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# For s390x see :ref:`cpu-topology-s390x`.
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#
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# The ids other than the node-id specify the position of the CPU
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# within the CPU topology (as defined by the machine property "smp",
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# thus see also type @SMPConfiguration)
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