Remove openvswitch runtime version checks

Specification for removing the runtime openvswitch version checks
and replacing them with an external script that specifically tests
the functionality required of openvswitch.

Change-Id: Id497149fd0ea32be31e32cb5308e4a9640e9e763

specs/juno/remove-openvswitch-version-check.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==========================================
+Remove openvswitch run-time version checks
+==========================================
+
+Launchpad blueprint:
+
+https://blueprints.launchpad.net/neutron/+spec/remove-openvswitch-version-check
+
+The current method of checking openvswitch and kernel versions for specific
+feature support is brittle, distro-specific and unsupportable.
+
+Problem description
+===================
+When VXLAN tunnels are enabled, openvswitch-using agents call ovs_lib code to
+check that the installed kernel and openvswitch versions are greater than a
+minimum specified version. Due to distro-specific differences in 1) the fields
+returned by modinfo and 2) the minimum required version due to the practice of
+backporting, this run-time checking method fails.
+
+In addition to VXLAN, other specific openvswitch features will need to be
+tested for in the future. As the number of features and versions and distros
+grows, checking version numbers becomes increasingly unsustainable and should
+not be done at agent start, but instead at package installation or deployment.
+
+Proposed change
+===============
+Trying to test features by version numbers across multiple distros is a fool's
+errand. It is much better to leave dependency management to the distros and
+packaging as they are the only ones who really know what version dependencies
+exist in their environment. The run-time checks should be completely removed
+from ovs_lib and the agents.
+
+If an attempt is made to use a specific openvswitch feature fails, a helpful
+error message suggesting that updating openvswitch *may* resolve the issue
+should be logged.
+
+To help deployment tools, a script will be created that attempts to use
+specific openvswitch features and exits 0 for success and 1 for failure. This
+way a deployment can be aborted, e.g. in devstack, if the feature is enabled
+but not available in the current openvswitch/kernel versions without trying
+to keep track of version numbers or incurring a messy run-time feature check
+at agent start.
+
+Alternatives
+------------
+Openvswitch does not have the ability to query whether it supports certain
+features, so the only way to reliably test whether a feature exists at runtime
+is to try to use the feature and see if it works. It would be possible, but
+unacceptably messy, to attempt to use all required OVS features at agent start
+and exit the agent on failure. Dependency management should be handled at
+packaging and/or deployment and not checked every time an agent is started by
+modifying system state.
+
+Data model impact
+-----------------
+N/A
+
+REST API impact
+---------------
+N/A
+
+Security impact
+---------------
+N/A
+
+Notifications impact
+--------------------
+N/A
+
+Other end user impact
+---------------------
+N/A
+
+Performance Impact
+------------------
+Negligible improvement in openvswitch-using agents due to removal of checks.
+
+Other deployer impact
+---------------------
+Existing deployments will be unaffected as they will already have the
+appropriate dependencies for their deployment.
+
+Deployments using distro packaging will be unaffected as the dependencies will
+already be handled by the packaging.
+
+Source installs, e.g. devstack, should run the feature test script to verify
+that an appropriate version of openvswitch has been installed on the system.
+
+Developer impact
+----------------
+Agents using openvswitch will no longer need to check openvswitch versions
+when using version-specific openvswitch features. Testing is simplified since
+the version-check feature is being removed and no longer requires testing.
+
+Devstack can also be modified to run the external feature test script before
+deployment and fail early in the case of a configuration/dependency mismatch.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+Primary assignee:
+  otherwiseguy
+
+Work Items
+----------
+* Remove runtime version checks and associated tests
+* Create the feature test script
+
+This will essentially be a refactoring of:
+
+  https://review.openstack.org/#/c/88121/
+
+into two pieces, the runtime check -> separate script and the existing code
+removal in that patch.
+
+Dependencies
+============
+N/A
+
+Testing
+=======
+As this implementation primarily removes code that should not exist, it also
+removes the need for testing the version checking code.
+
+Documentation Impact
+====================
+N/A
+
+References
+==========
+N/A