Merge "Re-propose using VirtIO PackedRing Configuration support for 2024.1"
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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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VirtIO PackedRing Configuration support
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==========================================
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https://blueprints.launchpad.net/nova/+spec/virtio-packedring-configuration-support
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This blueprint proposes to expose the LibVirt `packed` option that allows a
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guest to negotiate support for the VirtIO packed-ring feature. This blueprint
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is used to solicit community's input.
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Problem description
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===================
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VM using a Virtio-net paravirtual network device uses Virtual queues (virtqs)
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to send and receive data between the virtio-net driver and the virtual or
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physical backed. The VirtIO standard originally defined a single type of virtq
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called split-ring queue. The latest edition of the standard (v1.1) adds a
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different type of the virtq, called packed-ring queue. A different layout of
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queue elements allows to increase the performance in both virtual and physical
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backeds.
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Split-ring support is the default option in VirtIO. Backends supporting
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the packed-ring virtqs advertise this by setting the `VIRTIO_F_RING_PACKED`
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feature bit during the feature negotiation. A guest driver then chooses the
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virtq layout based on what it supports. As both options are identical features
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wise, and the packed-ring is more efficient, the latter is typically chosen.
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QEMU added support for the packed virtqs in v4.2 and LibVirt in v6.3. Qemu and
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LibVirt supports the packed-ring virtqs via the `packed` option. However, note
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that this option *does not* force the VM to use the packed-ring virtq. It acts
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as a mask, allowing the backed to advertise the support when set. The driver in
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the VM is still responsible for choosing the layout of virtqs.
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This blueprint proposes to add a Nova flavor extra_spec and Glance image
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property, that sets the `packed` option to `true` on the node. This way all VMs
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running on the node are allowed to choose the virtq layout based on what is
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offered by the backed, rather than being froced to use split-ring.
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Use Cases
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---------
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As an operator, I want to benefit from the increase in the virtio-net
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performance, by using a more efficient virtq structure.
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Proposed change
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===============
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* Add ``hw_virtio_packed_ring`` for image property and
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``hw:virtio_packed_ring`` for flavor extra specs.
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Users will control the packed virtqueue feature, and be able to disable
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it if desired.
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hw_virtio_packed_ring=true|false (default false)
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hw:virtio_packed_ring=true|false (default false)
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* Provide new compute ``COMPUTE_NET_VIRTIO_PACKED`` capablity trait.
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This trait can be required/forbidden by user. Nova-compute agent
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will automatically set this trait to the resource provider summary
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as far as current minimal libvirt support version by OpenStack is higher
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than feature required version 6.3.
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* This spec will update scheduling process. ALL_REQUEST_FILTERS will be
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extended with new filter packed_virtqueue_filter. It will update RequestSpec
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with new trait in case if image property or flavor extra_spec is enabled to
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avoid migration to the node without packed virtqueue feature support.
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.. _'libvirt format domain':
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https://libvirt.org/formatdomain.html#virtio-related-options
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Alternatives
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------------
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Leave as-is, operator will not have additional performance impact.
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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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VMs using virtio-net will see an increase in performance. The increase can be
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anywhere between 10/20% (see DPDK Intel Vhost/virtio perf. reports) and 75%
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(using Napatech SmartNICs).
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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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If operators upgrade their computes one by one, only new upgraded computes will
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support that feature for a while.
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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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justas_napa on IRC and Gerrit
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Additional assignees:
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dvo-plv on IRC and Gerrit
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Feature Liaison
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---------------
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* Sean Mooney (sean-k-mooney)
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Work Items
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----------
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* Add image property and flavor extra specs
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* Provide new compute capability trait
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* Update scheduling process
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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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New Unit and Functional tests will be added:
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* Verify image property and flavor extra spec options for correct
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configuration.
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* Verify nodes filtering by trait.
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* Verify correct Libvirt xml with driver packet option configuration.
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Documentation Impact
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====================
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Configuration options reference will require an update.
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References
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==========
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* VirtIO standard:
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https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html
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* LibVirt Domain XML reference
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https://libvirt.org/formatdomain.html#virtio-related-options
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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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* - 2023.2 Bobcat
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- Accepted
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* - 2024.1 Caracal
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- Reproposed
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