docs: introduce dedicated page about code provenance / sign-off

Currently we have a short paragraph saying that patches must include
a Signed-off-by line, and merely link to the kernel documentation.
The linked kernel docs have a lot of content beyond the part about
sign-off an thus are misleading/distracting to QEMU contributors.

This introduces a dedicated 'code-provenance' page in QEMU talking
about why we require sign-off, explaining the other tags we commonly
use, and what to do in some edge cases.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

3 files changed, 234 insertions, 22 deletions

diff --git a/docs/devel/code-provenance.rst b/docs/devel/code-provenance.rst
new file mode 100644
index 0000000..95b2dd3
--- /dev/null
+++ b/docs/devel/code-provenance.rst
@@ -0,0 +1,230 @@
+.. _code-provenance:
+
+Code provenance
+===============
+
+Certifying patch submissions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The QEMU community **mandates** all contributors to certify provenance of
+patch submissions they make to the project. To put it another way,
+contributors must indicate that they are legally permitted to contribute to
+the project.
+
+Certification is achieved with a low overhead by adding a single line to the
+bottom of every git commit::
+
+   Signed-off-by: YOUR NAME <YOUR@EMAIL>
+
+The addition of this line asserts that the author of the patch is contributing
+in accordance with the clauses specified in the
+`Developer's Certificate of Origin <https://developercertificate.org>`__:
+
+.. _dco:
+
+  Developer's Certificate of Origin 1.1
+
+  By making a contribution to this project, I certify that:
+
+  (a) The contribution was created in whole or in part by me and I
+      have the right to submit it under the open source license
+      indicated in the file; or
+
+  (b) The contribution is based upon previous work that, to the best
+      of my knowledge, is covered under an appropriate open source
+      license and I have the right under that license to submit that
+      work with modifications, whether created in whole or in part
+      by me, under the same open source license (unless I am
+      permitted to submit under a different license), as indicated
+      in the file; or
+
+  (c) The contribution was provided directly to me by some other
+      person who certified (a), (b) or (c) and I have not modified
+      it.
+
+  (d) I understand and agree that this project and the contribution
+      are public and that a record of the contribution (including all
+      personal information I submit with it, including my sign-off) is
+      maintained indefinitely and may be redistributed consistent with
+      this project or the open source license(s) involved.
+
+The name used with "Signed-off-by" does not need to be your legal name, nor
+birth name, nor appear on any government ID. It is the identity you choose to
+be known by in the community, but should not be anonymous, nor misrepresent
+whom you are.
+
+It is generally expected that the name and email addresses used in one of the
+``Signed-off-by`` lines, matches that of the git commit ``Author`` field.
+It's okay if you subscribe or contribute to the list via more than one
+address, but using multiple addresses in one commit just confuses
+things.
+
+If the person sending the mail is not one of the patch authors, they are
+nonetheless expected to add their own ``Signed-off-by`` to comply with the
+DCO clause (c).
+
+Multiple authorship
+~~~~~~~~~~~~~~~~~~~
+
+It is not uncommon for a patch to have contributions from multiple authors. In
+this scenario, git commits will usually be expected to have a ``Signed-off-by``
+line for each contributor involved in creation of the patch. Some edge cases:
+
+  * The non-primary author's contributions were so trivial that they can be
+    considered not subject to copyright. In this case the secondary authors
+    need not include a ``Signed-off-by``.
+
+    This case most commonly applies where QEMU reviewers give short snippets
+    of code as suggested fixes to a patch. The reviewers don't need to have
+    their own ``Signed-off-by`` added unless their code suggestion was
+    unusually large, but it is common to add ``Suggested-by`` as a credit
+    for non-trivial code.
+
+  * Both contributors work for the same employer and the employer requires
+    copyright assignment.
+
+    It can be said that in this case a ``Signed-off-by`` is indicating that
+    the person has permission to contribute from their employer who is the
+    copyright holder. It is nonetheless still preferable to include a
+    ``Signed-off-by`` for each contributor, as in some countries employees are
+    not able to assign copyright to their employer, and it also covers any
+    time invested outside working hours.
+
+When multiple ``Signed-off-by`` tags are present, they should be strictly kept
+in order of authorship, from oldest to newest.
+
+Other commit tags
+~~~~~~~~~~~~~~~~~
+
+While the ``Signed-off-by`` tag is mandatory, there are a number of other tags
+that are commonly used during QEMU development:
+
+ * **``Reviewed-by``**: when a QEMU community member reviews a patch on the
+   mailing list, if they consider the patch acceptable, they should send an
+   email reply containing a ``Reviewed-by`` tag. Subsystem maintainers who
+   review a patch should add this even if they are also adding their
+   ``Signed-off-by`` to the same commit.
+
+ * **``Acked-by``**: when a QEMU subsystem maintainer approves a patch that
+   touches their subsystem, but intends to allow a different maintainer to
+   queue it and send a pull request, they would send a mail containing a
+   ``Acked-by`` tag. Where a patch touches multiple subsystems, ``Acked-by``
+   only implies review of the maintainers' own areas of responsibility. If a
+   maintainer wants to indicate they have done a full review they should use
+   a ``Reviewed-by`` tag.
+
+ * **``Tested-by``**: when a QEMU community member has functionally tested the
+   behaviour of the patch in some manner, they should send an email reply
+   containing a ``Tested-by`` tag.
+
+ * **``Reported-by``**: when a QEMU community member reports a problem via the
+   mailing list, or some other informal channel that is not the issue tracker,
+   it is good practice to credit them by including a ``Reported-by`` tag on
+   any patch fixing the issue. When the problem is reported via the GitLab
+   issue tracker, however, it is sufficient to just include a link to the
+   issue.
+
+ * **``Suggested-by``**: when a reviewer or other 3rd party makes non-trivial
+   suggestions for how to change a patch, it is good practice to credit them
+   by including a ``Suggested-by`` tag.
+
+Subsystem maintainer requirements
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When a subsystem maintainer accepts a patch from a contributor, in addition to
+the normal code review points, they are expected to validate the presence of
+suitable ``Signed-off-by`` tags.
+
+At the time they queue the patch in their subsystem tree, the maintainer
+**must** also then add their own ``Signed-off-by`` to indicate that they have
+done the aforementioned validation. This is in addition to any of their own
+``Reviewed-by`` tags the subsystem maintainer may wish to include.
+
+When the maintainer modifies the patch after pulling into their tree, they
+should record their contribution.  This is typically done via a note in the
+commit message, just prior to the maintainer's ``Signed-off-by``::
+
+    Signed-off-by: Cory Contributor <cory.contributor@example.com>
+    [Comment rephrased for clarity]
+    Signed-off-by: Mary Maintainer <mary.maintainer@mycorp.test>
+
+
+Tools for adding ``Signed-off-by``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are a variety of ways tools can support adding ``Signed-off-by`` tags
+for patches, avoiding the need for contributors to manually type in this
+repetitive text each time.
+
+git commands
+^^^^^^^^^^^^
+
+When creating, or amending, a commit the ``-s`` flag to ``git commit`` will
+append a suitable line matching the configured git author details.
+
+If preparing patches using the ``git format-patch`` tool, the ``-s`` flag can
+be used to append a suitable line in the emails it creates, without modifying
+the local commits. Alternatively to modify all the local commits on a branch::
+
+  git rebase master -x 'git commit --amend --no-edit -s'
+
+emacs
+^^^^^
+
+In the file ``$HOME/.emacs.d/abbrev_defs`` add:
+
+.. code:: elisp
+
+  (define-abbrev-table 'global-abbrev-table
+    '(
+      ("8rev" "Reviewed-by: YOUR NAME <your@email.addr>" nil 1)
+      ("8ack" "Acked-by: YOUR NAME <your@email.addr>" nil 1)
+      ("8test" "Tested-by: YOUR NAME <your@email.addr>" nil 1)
+      ("8sob" "Signed-off-by: YOUR NAME <your@email.addr>" nil 1)
+     ))
+
+with this change, if you type (for example) ``8rev`` followed by ``<space>``
+or ``<enter>`` it will expand to the whole phrase.
+
+vim
+^^^
+
+In the file ``$HOME/.vimrc`` add::
+
+  iabbrev 8rev Reviewed-by: YOUR NAME <your@email.addr>
+  iabbrev 8ack Acked-by: YOUR NAME <your@email.addr>
+  iabbrev 8test Tested-by: YOUR NAME <your@email.addr>
+  iabbrev 8sob Signed-off-by: YOUR NAME <your@email.addr>
+
+with this change, if you type (for example) ``8rev`` followed by ``<space>``
+or ``<enter>`` it will expand to the whole phrase.
+
+Re-starting abandoned work
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For a variety of reasons there are some patches that get submitted to QEMU but
+never merged. An unrelated contributor may decide (months or years later) to
+continue working from the abandoned patch and re-submit it with extra changes.
+
+The general principles when picking up abandoned work are:
+
+ * Continue to credit the original author for their work, by maintaining their
+   original ``Signed-off-by``
+ * Indicate where the original patch was obtained from (mailing list, bug
+   tracker, author's git repo, etc) when sending it for review
+ * Acknowledge the extra work of the new contributor by including their
+   ``Signed-off-by`` in the patch in addition to the orignal author's
+ * Indicate who is responsible for what parts of the patch. This is typically
+   done via a note in the commit message, just prior to the new contributor's
+   ``Signed-off-by``::
+
+    Signed-off-by: Some Person <some.person@example.com>
+    [Rebased and added support for 'foo']
+    Signed-off-by: New Person <new.person@mycorp.test>
+
+In complicated cases, or if otherwise unsure, ask for advice on the project
+mailing list.
+
+It is also recommended to attempt to contact the original author to let them
+know you are interested in taking over their work, in case they still intended
+to return to the work, or had any suggestions about the best way to continue.
diff --git a/docs/devel/index-process.rst b/docs/devel/index-process.rst
index cb7c664..5807752 100644
--- a/docs/devel/index-process.rst
+++ b/docs/devel/index-process.rst
@@ -13,6 +13,7 @@ Notes about how to interact with the community and how and where to submit patch
    maintainers
    style
    submitting-a-patch
+   code-provenance
    trivial-patches
    stable-process
    submitting-a-pull-request
diff --git a/docs/devel/submitting-a-patch.rst b/docs/devel/submitting-a-patch.rst
index 65c6407..f7917b8 100644
--- a/docs/devel/submitting-a-patch.rst
+++ b/docs/devel/submitting-a-patch.rst
@@ -344,28 +344,9 @@ Patch emails must include a ``Signed-off-by:`` line
 
 Your patches **must** include a Signed-off-by: line. This is a hard
 requirement because it's how you say "I'm legally okay to contribute
-this and happy for it to go into QEMU". The process is modelled after
-the `Linux kernel
-<http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/SubmittingPatches?id=f6f94e2ab1b33f0082ac22d71f66385a60d8157f#n297>`__
-policy.
-
-If you wrote the patch, make sure your "From:" and "Signed-off-by:"
-lines use the same spelling. It's okay if you subscribe or contribute to
-the list via more than one address, but using multiple addresses in one
-commit just confuses things. If someone else wrote the patch, git will
-include a "From:" line in the body of the email (different from your
-envelope From:) that will give credit to the correct author; but again,
-that author's Signed-off-by: line is mandatory, with the same spelling.
-
-The name used with "Signed-off-by" does not need to be your legal name,
-nor birth name, nor appear on any government ID. It is the identity you
-choose to be known by in the community, but should not be anonymous,
-nor misrepresent whom you are.
-
-There are various tooling options for automatically adding these tags
-include using ``git commit -s`` or ``git format-patch -s``. For more
-information see `SubmittingPatches 1.12
-<http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/SubmittingPatches?id=f6f94e2ab1b33f0082ac22d71f66385a60d8157f#n297>`__.
+this and happy for it to go into QEMU". For full guidance, read the
+:ref:`code-provenance` documentation.
+
 
 .. _include_a_meaningful_cover_letter: