# Backporting patches

Note

This is an advanced topic for developers and maintainers. Readers should familiarize themselves with building and running Open vSwitch, with the git tool, and with the Open vSwitch patch submission process.

The backporting of patches from one git tree to another takes multiple forms within Open vSwitch, but is broadly applied in the following fashion:

- Contributors submit their proposed changes to the latest development branch

- Contributors and maintainers provide feedback on the patches

- When the change is satisfactory, maintainers apply the patch to the development branch.

- Maintainers backport changes from a development branch to release branches.

With regards to Open vSwitch user space code and code that does not comprise the Linux datapath and compat code, the development branch is master in the Open vSwitch repository. Patches are applied first to this branch, then to the most recent branch-X.Y, then earlier branch-X.Z, and so on. The most common kind of patch in this category is a bugfix which affects master and other branches.

For Linux datapath code, the primary development branch is in the net-next tree as described in the section below, and patch discussion occurs on the netdev mailing list. Patches are first applied to the upstream branch by the networking maintainer, then the contributor backports the patch to the Open vSwitch master development branch. Patches in this category may include features which have been applied upstream, or bugfixes to the Open vSwitch datapath code. For bugfixes, the patches subsequently follow the regular Open vSwitch process as described above to reach older branches.

## Changes to userspace components

Patches which are fixing bugs should be considered for backporting from master to release branches. Open vSwitch contributors submit their patches targeted to the master branch, using the `Fixes` tag described in Submitting Patches. The maintainer first applies the patch to master, then backports the patch to each older affected tree, as far back as it goes or at least to all currently supported branches. This is usually each branch back to the oldest maintained LTS release branch or the last 4 release branches if the oldest LTS is newer.

If the fix only affects a particular branch and not master, contributors should submit the change with the target branch listed in the subject line of the patch. Contributors should list all versions that the bug affects. The `git format-patch` argument `--subject-prefix` may be used when posting the patch, for example:

    $ git format-patch HEAD --subject-prefix="PATCH branch-2.7"

If a maintainer is backporting a change to older branches and the backport is not a trivial cherry-pick, then the maintainer may opt to submit the backport for the older branch on the mailing list for further review. This should be done in the same manner as described above.

## Changes to Linux kernel components

The Linux kernel components in Open vSwitch go through initial review in the upstream Linux netdev community before they go into the Open vSwitch tree. As such, backports from upstream to the Open vSwitch tree may include bugfixes or new features. The netdev-FAQ describes the general process for merging patches to the upstream Linux tree.

To keep track of the changes which are made upstream against the changes which have been backported to the Open vSwitch tree, backports should be done in the order that they are applied to the upstream net-next tree. For example, if the git history in `linux/net/openvswitch/` in the net-next tree lists patches A, B and C that were applied (in that order), then the backports of these patches to `openvswitch/datapath/` should be done submitted in the order A, B, then C.

Patches that are proposed against the Open vSwitch tree, including backports, should follow the guidelines described in Submitting Patches. Ideally, a series which backports new functionality would also include a series of patches for the userspace components which show how to use the new functionality, and include tests to validate the behaviour. However, in the interests of keeping the Open vSwitch tree in sync with upstream net-next, contributors may send Open vSwitch kernel module changes independently of userspace changes.

### How to backport kernel patches

These instructions only apply to Open vSwitch releases 2.17 and older. As of Open vSwitch branch 3.0 the Open vSwitch kernel module is no longer supported and only the Linux openvswitch kernel module is used. In the case of Open vSwitch releases 2.17 and older, kernel backports may be required for bux fixes and feature implementation so these instructions are preserved for that reason.

First, the patch should be submitted upstream to netdev. When the patch has been applied to net-next, it is ready to be backported. Starting from the Linux tree, use `git format-patch` to format each patch that should be backported. For each of these patches, they may only include changes to `linux/net/openvswitch/`, or they may include changes to other directories. Depending on which files the patch touches, the backport may be easier or more difficult to undertake.

Start by formatting the relevant patches from the Linux tree. For example, to format the last 5 patches to `net/openvswitch`, going back from OVS commit `1234c0ffee5`, placing them into `/tmp/`:

    $ git format-patch -5 1234c0ffee5 net/openvswitch/ -o /tmp

Next, change into the Open vSwitch directory and apply the patch:

    $ git am -p3 --reject --directory=datapath/ <patch>

If this is successful, proceed to the next patch:

    $ git am --continue

If this is unsuccessful, the above command applies all changes that it can to the working tree, and leaves rejected hunks in corresponding *.rej files. Proceed by using `git diff` to identify the changes, and edit the files so that the hunk matches what the file looks like when the corresponding commit is checked out in the linux tree. When all hunks are fixed, add the files to the index using `git add`.

If the patch only changes filepaths under `linux/net/openvswitch`, then most likely the patch is fully backported. At this point, review the patch’s changes and compare with the latest upstream code for the modified functions. Occasionally, there may be bugs introduced in a particular patch which were fixed in a later patch upstream. To prevent breakage in the OVS tree, consider rolling later bugfixes into the current patch - particularly if they are small, clear bugfixes in the logic of this patch. Then proceed to the next patch using `git am --continue`. If you made any changes to the patch compared with the original version, describe the changes in the commit message.

If the changes affects other paths, then you may also need to backport function definitions from the upstream tree into the `datapath/linux/compat` directory. First, attempt to compile the datapath. If this is successful, then most likely there is no further work required. As per the previous paragraph, consider reviewing and backporting any minor fixes to this code if applicable, then proceed to the next patch using `git am --continue`.

If compilation fails, the compiler will show which functions are missing or broken. Typically this should match with some function definitions provided in the patch file. The following command will attempt to apply all such changes from the patch into the `openvswitch/datapath/linux/compat` directory; Like the previous `git am` command above, it may succeed or fail. If it succeeds, review the patch and proceed to the next patch using `git am --continue`.

    $ git am -p3 --reject --directory='datapath/linux/compat/' <patch>

For each conflicting hunk, attempt to resolve the change so that the function reflects what the function looks like in the upstream Linux tree. After resolving these changes, compile the changes, add the modified files to the index using `git add`, review the patch, and proceed to the next patch using `git am --continue`.

### Submission

Once the patches are all assembled and working on the Open vSwitch tree, they need to be formatted again using `git format-patch`. The common format for commit messages for Linux backport patches is as follows:

    datapath: Remove incorrect WARN_ONCE().

    Upstream commit:
        commit c6b2aafffc6934be72d96855c9a1d88970597fbc
        Author: Jarno Rajahalme <jarno@ovn.org>
        Date:   Mon Aug 1 19:08:29 2016 -0700

        openvswitch: Remove incorrect WARN_ONCE().

        ovs_ct_find_existing() issues a warning if an existing conntrack entry
        classified as IP_CT_NEW is found, with the premise that this should
        not happen.  However, a newly confirmed, non-expected conntrack entry
        remains IP_CT_NEW as long as no reply direction traffic is seen.  This
        has resulted into somewhat confusing kernel log messages.  This patch
        removes this check and warning.

        Fixes: 289f2253 ("openvswitch: Find existing conntrack entry after upcall.")
        Suggested-by: Joe Stringer <joe@ovn.org>
        Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
        Acked-by: Joe Stringer <joe@ovn.org>

    Signed-off-by: Jarno Rajahalme <jarno@ovn.org>

The upstream commit SHA should be the one that appears in Linus’ tree so that reviewers can compare the backported patch with the one upstream. Note that the subject line for the backported patch replaces the original patch’s `openvswitch` prefix with `datapath`. Patches which only affect the `datapath/linux/compat` directory should be prefixed with `compat`.

The contents of a backport should be equivalent to the changes made by the original patch; explain any variations from the original patch in the commit message - For instance if you rolled in a bugfix. Reviewers will verify that the changes made by the backport patch are the same as the changes made in the original commit which the backport is based upon. Patch submission should otherwise follow the regular steps described in Submitting Patches. In particular, if performing kernel patch backports, pay attention to Datapath testing.