High Precision Event Timer (HPET) on x86 guests
Support HPET for x86 guests in the libvirt driver. blueprint: support-hpet-on-guest Co-Authored-By: Chris Friesen <chris.friesen@windriver.com> Change-Id: I26c5f5386b99f88dcc7f6ea4629a61f93084b3e7 Signed-off-by: Jack Ding <jack.ding@windriver.com>
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Eric Fried
Matt Riedemann
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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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Support High Precision Event Timer (HPET) on x86 guests
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=======================================================
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https://blueprints.launchpad.net/nova/+spec/support-hpet-on-guest
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Problem description
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===================
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Use Cases
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---------
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As an end user looking to migrate an existing appliance to run in a cloud
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environment I would like to be able to request a guest with HPET so that I can
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share common code between my virtualized and physical products.
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As an operator I would like to support onboarding legacy VNFs for my telco
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customers where a guest image cannot be modified to work without a HPET.
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Proposed change
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===============
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End users can indicate their desire to have HPET in the guest by specifying a
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image property ``hw_time_hpet=True``.
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Setting the new image property to "True" would only be guaranteed to be valid
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in combination with ``hypervisor_type=qemu`` and either ``architecture=i686``
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or ``architecture=x86_64``.
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.. note:: A corresponding flavor extra spec will not be introduced since
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enabling HPET is really a per-image concern rather than a resource concern
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for capacity planning.
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A few options to use Traits were considered as described in the next section,
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but we end up choosing the simpler approach due to the following reasons:
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1) HPET is provided by qemu via emulation, so there are no security
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implications as there are already better clock sources available.
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2) The HPET was turned off by default purely because of issues with time
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drifting on Windows guests. (See nova commit ba3fd16605.)
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3) The emulated HPET device is unconditionally available on all versions of
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libvirt/qemu supported by OpenStack.
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4) The HPET device is only supported for x86 architectures, so in a cloud with
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a mix of architectures the image would have to be specific to ensure the
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instance is scheduled on an x86 host.
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5) Initially we would only support enabling HPET on qemu. Specifying the
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hypervisor type will ensure the instance is scheduled on a host using the
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qemu hypervisor. It would be possible to extend this to other hypervisors
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as well if applicable (vmware supports the ability to enable/disable HPET,
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I think), and which ones are supported could be documented in the "useful
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image properties" documentation.
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Alternatives
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------------
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The following options to use Trait were considered, but ultimatedly we chose
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a simpler approach without using Trait.
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Explicit Trait, Implicit Config
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Operators can indicate their desire to have HPET in the guest by specifying a
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placement trait ``trait:COMPUTE_TIME_HPET=required`` in the flavor extra-specs.
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End users can indicate their desire to have HPET in the guest by uploading
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their own images with the same trait.
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Existing nova scheduler code picks up the trait and passes it to
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``GET /allocation_candidates``.
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Once scheduled to a compute node, the virt driver looks for
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``trait:COMPUTE_TIME_HPET=required`` in the flavor/image or
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``trait*:COMPUTE_TIME_HPET=required`` for numbered request group in flavor and
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uses that as its cue to enable HPET on the guest.
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If we do get down to the virt driver and the trait is set, and the driver for
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whatever reason (e.g. value(s) wrong in the flavor; wind up on a host that
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doesn't support HPET etc.) determines it's not capable of flipping the switch,
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it should fail. [1]_
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**CON:** We're using a trait to effect guest configuration.
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Explicit Config, Implicit Trait
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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* Operator specifies extra spec ``hw:hpet=True`` in the flavor.
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* Nova recognizes this as a known special case and adds
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``required=COMPUTE_TIME_HPET`` to the ``GET /allocation_candidates`` query.
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* The driver uses the ``hw:hpet=True`` extra spec as its cue to enable HPET on
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the guest.
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**CON:** The implicit transformation of a special extra spec into
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placement-isms is arcane. This wouldn't be the only instance of this; we would
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need to organize the "special" extra specs in the code for maintainability, and
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document them thoroughly.
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Explicit Config, Explicit Trait
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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* Operator specifies **both** extra specs, ``hw:hpet=True`` and
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``trait:COMPUTE_TIME_HPET=required``, in the flavor.
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* Existing nova scheduler code picks up the latter and passes it to ``GET
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/allocation_candidates``.
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* The driver uses the ``hw:hpet=True`` extra spec as its cue to enable HPET on
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the guest.
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**CON:** The operator has to remember to set both extra specs, which is kind of
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gross UX. (If she forgets ``hw:hpet=True``, she ends up with HPET off; if she
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forgets ``trait:COMPUTE_TIME_HPET=required``, she can end up with late-failing
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NoValidHosts.)
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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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Negligible.
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Other deployer impact
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---------------------
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None
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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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The new image property will only work reliably after all nodes have been
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upgraded.
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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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jackding
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Other contributors:
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jaypipes, efried
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Work Items
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----------
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* libvirt driver changes to support HPET
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Dependencies
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============
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None
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Testing
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=======
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Will add unit tests.
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Documentation Impact
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====================
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Update User Documentation for image properties [2]_.
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References
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==========
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.. [1] http://lists.openstack.org/pipermail/openstack-dev/2018-October/135446.html
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.. [2] https://docs.openstack.org/glance/latest/admin/useful-image-properties.html
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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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* - Stein
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- Introduced
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