Merge "Add maxphysaddr support for Libvirt"
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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==========================================
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Add maxphysaddr support for Libvirt
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==========================================
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https://blueprints.launchpad.net/nova/+spec/libvirt-maxphysaddr-support
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This blueprint propose new flavor extra_specs to control the physical
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address bits of vCPUs in Libvirt guests.
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Problem description
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===================
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When booting a guest with 1TB+ RAM, the default physical address bits are
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too small and the boot fails [1]_. So a knob is needed to specify the
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appropriate physical address bits.
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Use Cases
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---------
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Booting a guest with large RAM.
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Proposed change
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===============
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In Libvirt v8.7.0+ and QEMU v2.7.0+, physical address bits can be specified
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with following XML elements [2]_ [3]_. The former means to adopt any physical
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address bits, the latter means to adopt the physical address bits of the
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host CPU.
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- ``<maxphysaddr mode='emulate' bits='42'/>``
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- ``<maxphysaddr mode='passthrough'/>``
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Flavor extra_specs
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-----------------------------------------------
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Here I suggest the following two flavor extra_specs.
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Of course, if these are omitted, the behavior is the same as before.
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- ``hw:maxphysaddr_mode`` can be either ``emulate`` or ``passthrough``.
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- ``hw:maxphysaddr_bits`` takes a positive integer value.
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Only meaningful and must be specified if ``hw:maxphysaddr_mode=emulate``.
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Nova scheduler changes
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----------------------
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Nova scheduler also needs to be modified to take these two properties
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into account.
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There can be a mix of supported and unsupported hosts depending
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on Libvirt and QEMU versions. So add new traits
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``COMPUTE_ADDRESS_SPACE_PASSTHROUGH`` and ``COMPUTE_ADDRESS_SPACE_EMULATED``
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to check the scheduled host supports this feature.
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``trait:COMPUTE_ADDRESS_SPACE_PASSTHROUGH=required`` is automatically added
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if ``hw:maxphysaddr_mode=passthrough`` is specified in flavor extra_specs.
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And same for ``hw:maxphysaddr_mode=emulate``.
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Passthrough and emulate modes have different properties. So let's consider
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the two separately.
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The case of ``hw:maxphysaddr_mode=passthrough``. In this case,
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``cpu_mode=host-passthrough`` is a requirement, which is already taken
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into account in nova scheduling, and no additional modifications are
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required in this proposal. It is not guaranteed whether the instance
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can be migrated by nova. So the admin needs to make sure that targets
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of cold and live migration have similar hardware and software.
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This restriction is similar for ``cpu_mode=host-passthrough``.
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The case of ``hw:maxphysaddr_mode=emulate``. In nova scheduling,
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it is necessary to check that the hypervisor supports at least
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``hw:maxphysaddr_bits``. The maximum number of bits supported by
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hypervisor can be obtained by using libvirt capabilities [4]_. Therefore,
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``ComputeCapabilitiesFilter`` can be used to compare the number of bits in
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scheduling. For example, this can be accomplished by adding
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``capabilities:cpu_info:maxphysaddr:bits>=42`` automatically.
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Cold migration and live migration can also be realized with this filter
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and ``COMPUTE_ADDRESS_SPACE_EMULATED`` trait.
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So the overall flavor extra_specs look like the following::
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openstack flavor set <flavor> \
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--property hw:maxphysaddr_mode=emulate \
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--property hw:maxphysaddr_bits=42
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.. note:: Since ComputeCapabilitiesFilter only supports flavor extra_specs
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and not image properties [5]_, this proposal is out of scope for
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image properties.
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Alternatives
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------------
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Before the ``maxphysaddr`` option was introduced into Libvirt, it was specified
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as a workaround with the QEMU comanndline parameter. But this alternative is
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not allowed in nova.
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Also, some Linux distributions may have machine types with
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``host-phys-bits=true`` [6]_. For example, ``pc-i440fx-bionic-hpb`` and
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``pc-q35-bionic-hpb``. However, this alternative has following two issues and
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cannot be adopted for general-purpose use cases.
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- Ubuntu package maintainers are applying a patch to QEMU [7]_. It means this
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is not included in vanilla QEMU and is not available in other distributions.
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- This is only the case for ``hw:maxphysaddr_mode=passthrough`` and does not
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include ``hw:maxphysaddr_mode=emulate``. Since
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``hw:maxphysaddr_mode=passthrough`` requires ``cpu_mode=host-passthrough``
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to be used [8]_, this alternative cannot be used with ``cpu_mode=custom``
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or ``cpu_mode=host-model``. So, this alternative is not sufficient for
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a cloud with many different CPU models.
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As for scheduling, placement does not currently support numeric traits,
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so the maximum number of bits supported by hypervisor cannot be checked
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by this mechanism.
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Data model impact
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-----------------
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None
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REST API impact
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---------------
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None
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Security impact
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---------------
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None
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Notifications impact
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--------------------
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None
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Other end user impact
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---------------------
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None
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Performance Impact
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------------------
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None
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Other deployer impact
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---------------------
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Operators should specify appropriate flavor extra_specs as needed.
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Developer impact
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----------------
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None
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Upgrade impact
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--------------
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As described earlier, the new traits ``COMPUTE_ADDRESS_SPACE_PASSTHROUGH`` and
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``COMPUTE_ADDRESS_SPACE_EMULATED`` signal if the upgraded compute nodes support
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this feature.
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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nmiki
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Other contributors:
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None
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Feature Liaison
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---------------
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Feature liaison:
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Liaison Needed
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Work Items
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----------
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* Add new guest configs
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* Add new fileds in nova/api/validation/extra_specs/hw.py
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* Add new fields in LibvirtConfigCPU in nova/virt/livbirt/config.py
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* Add new traits to check Libvirt and QEMU versions
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* Add new field ``maxphysaddr`` to ``cpu_info`` in nova/virt/libvirt/driver.py
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* Add docs and release notes for new flavor extra_specs
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Dependencies
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============
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Libivrt v8.7.0+.
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QEMU v2.7.0+.
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Testing
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=======
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Add the following unit tests:
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- check that proposed flavor extra_specs are properly validated
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- check that intended XML elements are output
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- check that traits are properly added and used
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- check that new field in ``ComputeCapabilitiesFilter`` is property
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added and used
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Documentation Impact
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====================
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For operators, the documentation describes what proposed flavor extra_specs
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mean and how they should be set.
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References
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==========
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.. [1] https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1769053
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.. [2] https://libvirt.org/news.html#v8-7-0-2022-09-01
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.. [3] https://github.com/libvirt/libvirt/commit/1c1a7cdd4096c59fb0c374529e1e5aea8d43ee9c
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.. [4] https://libvirt.org/formatcaps.html#examples
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.. [5] https://docs.openstack.org/nova/latest/admin/scheduling.html#computecapabilitiesfilter
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.. [6] https://cpaelzer.github.io/blogs/005-guests-bigger-than-1tb/
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.. [7] https://git.launchpad.net/~paelzer/ubuntu/+source/qemu/commit/?id=6ba8b5c843d405e1b067dc8b98ecb8545af78a2b
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.. [8] https://github.com/libvirt/libvirt/blob/v8.7.0/src/qemu/qemu_validate.c#L346-L351
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History
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=======
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.. list-table:: Revisions
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:header-rows: 1
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* - Release Name
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- Description
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* - 2023.1 Antelope
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- Introduced
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