xref: /linux-6.8/Documentation/process/maintainer-kvm-x86.rst

.. SPDX-License-Identifier: GPL-2.0

KVM x86
=======

Foreword
--------
KVM strives to be a welcoming community; contributions from newcomers are
valued and encouraged.  Please do not be discouraged or intimidated by the
length of this document and the many rules/guidelines it contains.  Everyone
makes mistakes, and everyone was a newbie at some point.  So long as you make
an honest effort to follow KVM x86's guidelines, are receptive to feedback,
and learn from any mistakes you make, you will be welcomed with open arms, not
torches and pitchforks.

TL;DR
-----
Testing is mandatory.  Be consistent with established styles and patterns.

Trees
-----
KVM x86 is currently in a transition period from being part of the main KVM
tree, to being "just another KVM arch".  As such, KVM x86 is split across the
main KVM tree, ``git.kernel.org/pub/scm/virt/kvm/kvm.git``, and a KVM x86
specific tree, ``github.com/kvm-x86/linux.git``.

Generally speaking, fixes for the current cycle are applied directly to the
main KVM tree, while all development for the next cycle is routed through the
KVM x86 tree.  In the unlikely event that a fix for the current cycle is routed
through the KVM x86 tree, it will be applied to the ``fixes`` branch before
making its way to the main KVM tree.

Note, this transition period is expected to last quite some time, i.e. will be
the status quo for the foreseeable future.

Branches
~~~~~~~~
The KVM x86 tree is organized into multiple topic branches.  The purpose of
using finer-grained topic branches is to make it easier to keep tabs on an area
of development, and to limit the collateral damage of human errors and/or buggy
commits, e.g. dropping the HEAD commit of a topic branch has no impact on other
in-flight commits' SHA1 hashes, and having to reject a pull request due to bugs
delays only that topic branch.

All topic branches, except for ``next`` and ``fixes``, are rolled into ``next``
via a Cthulhu merge on an as-needed basis, i.e. when a topic branch is updated.
As a result, force pushes to ``next`` are common.

Lifecycle
~~~~~~~~~
Fixes that target the current release, a.k.a. mainline, are typically applied
directly to the main KVM tree, i.e. do not route through the KVM x86 tree.

Changes that target the next release are routed through the KVM x86 tree.  Pull
requests (from KVM x86 to main KVM) are sent for each KVM x86 topic branch,
typically the week before Linus' opening of the merge window, e.g. the week
following rc7 for "normal" releases.  If all goes well, the topic branches are
rolled into the main KVM pull request sent during Linus' merge window.

The KVM x86 tree doesn't have its own official merge window, but there's a soft
close around rc5 for new features, and a soft close around rc6 for fixes (for
the next release; see above for fixes that target the current release).

Timeline
~~~~~~~~
Submissions are typically reviewed and applied in FIFO order, with some wiggle
room for the size of a series, patches that are "cache hot", etc.  Fixes,
especially for the current release and or stable trees, get to jump the queue.
Patches that will be taken through a non-KVM tree (most often through the tip
tree) and/or have other acks/reviews also jump the queue to some extent.

Note, the vast majority of review is done between rc1 and rc6, give or take.
The period between rc6 and the next rc1 is used to catch up on other tasks,
i.e. radio silence during this period isn't unusual.

Pings to get a status update are welcome, but keep in mind the timing of the
current release cycle and have realistic expectations.  If you are pinging for
acceptance, i.e. not just for feedback or an update, please do everything you
can, within reason, to ensure that your patches are ready to be merged!  Pings
on series that break the build or fail tests lead to unhappy maintainers!

Development
-----------

Base Tree/Branch
~~~~~~~~~~~~~~~~
Fixes that target the current release, a.k.a. mainline, should be based on
``git://git.kernel.org/pub/scm/virt/kvm/kvm.git master``.  Note, fixes do not
automatically warrant inclusion in the current release.  There is no singular
rule, but typically only fixes for bugs that are urgent, critical, and/or were
introduced in the current release should target the current release.

Everything else should be based on ``kvm-x86/next``, i.e. there is no need to
select a specific topic branch as the base.  If there are conflicts and/or
dependencies across topic branches, it is the maintainer's job to sort them
out.

The only exception to using ``kvm-x86/next`` as the base is if a patch/series
is a multi-arch series, i.e. has non-trivial modifications to common KVM code
and/or has more than superficial changes to other architectures' code.  Multi-
arch patch/series should instead be based on a common, stable point in KVM's
history, e.g. the release candidate upon which ``kvm-x86 next`` is based.  If
you're unsure whether a patch/series is truly multi-arch, err on the side of
caution and treat it as multi-arch, i.e. use a common base.

Coding Style
~~~~~~~~~~~~
When it comes to style, naming, patterns, etc., consistency is the number one
priority in KVM x86.  If all else fails, match what already exists.

With a few caveats listed below, follow the tip tree maintainers' preferred
:ref:`maintainer-tip-coding-style`, as patches/series often touch both KVM and
non-KVM x86 files, i.e. draw the attention of KVM *and* tip tree maintainers.

Using reverse fir tree, a.k.a. reverse Christmas tree or reverse XMAS tree, for
variable declarations isn't strictly required, though it is still preferred.

Except for a handful of special snowflakes, do not use kernel-doc comments for
functions.  The vast majority of "public" KVM functions aren't truly public as
they are intended only for KVM-internal consumption (there are plans to
privatize KVM's headers and exports to enforce this).

Comments
~~~~~~~~
Write comments using imperative mood and avoid pronouns.  Use comments to
provide a high level overview of the code, and/or to explain why the code does
what it does.  Do not reiterate what the code literally does; let the code
speak for itself.  If the code itself is inscrutable, comments will not help.

SDM and APM References
~~~~~~~~~~~~~~~~~~~~~~
Much of KVM's code base is directly tied to architectural behavior defined in
Intel's Software Development Manual (SDM) and AMD's Architecture Programmer’s
Manual (APM).  Use of "Intel's SDM" and "AMD's APM", or even just "SDM" or
"APM", without additional context is a-ok.

Do not reference specific sections, tables, figures, etc. by number, especially
not in comments.  Instead, if necessary (see below), copy-paste the relevant
snippet and reference sections/tables/figures by name.  The layouts of the SDM
and APM are constantly changing, and so the numbers/labels aren't stable.

Generally speaking, do not explicitly reference or copy-paste from the SDM or
APM in comments.  With few exceptions, KVM *must* honor architectural behavior,
therefore it's implied that KVM behavior is emulating SDM and/or APM behavior.
Note, referencing the SDM/APM in changelogs to justify the change and provide
context is perfectly ok and encouraged.

Shortlog
~~~~~~~~
The preferred prefix format is ``KVM: <topic>:``, where ``<topic>`` is one of::

  - x86
  - x86/mmu
  - x86/pmu
  - x86/xen
  - selftests
  - SVM
  - nSVM
  - VMX
  - nVMX

**DO NOT use x86/kvm!**  ``x86/kvm`` is used exclusively for Linux-as-a-KVM-guest
changes, i.e. for arch/x86/kernel/kvm.c.  Do not use file names or complete file
paths as the subject/shortlog prefix.

Note, these don't align with the topics branches (the topic branches care much
more about code conflicts).

All names are case sensitive!  ``KVM: x86:`` is good, ``kvm: vmx:`` is not.

Capitalize the first word of the condensed patch description, but omit ending
punctionation.  E.g.::

    KVM: x86: Fix a null pointer dereference in function_xyz()

not::

    kvm: x86: fix a null pointer dereference in function_xyz.

If a patch touches multiple topics, traverse up the conceptual tree to find the
first common parent (which is often simply ``x86``).  When in doubt,
``git log path/to/file`` should provide a reasonable hint.

New topics do occasionally pop up, but please start an on-list discussion if
you want to propose introducing a new topic, i.e. don't go rogue.

See :ref:`the_canonical_patch_format` for more information, with one amendment:
do not treat the 70-75 character limit as an absolute, hard limit.  Instead,
use 75 characters as a firm-but-not-hard limit, and use 80 characters as a hard
limit.  I.e. let the shortlog run a few characters over the standard limit if
you have good reason to do so.

Changelog
~~~~~~~~~
Most importantly, write changelogs using imperative mood and avoid pronouns.

See :ref:`describe_changes` for more information, with one amendment: lead with
a short blurb on the actual changes, and then follow up with the context and
background.  Note!  This order directly conflicts with the tip tree's preferred
approach!  Please follow the tip tree's preferred style when sending patches
that primarily target arch/x86 code that is _NOT_ KVM code.

Stating what a patch does before diving into details is preferred by KVM x86
for several reasons.  First and foremost, what code is actually being changed
is arguably the most important information, and so that info should be easy to
find. Changelogs that bury the "what's actually changing" in a one-liner after
3+ paragraphs of background make it very hard to find that information.

For initial review, one could argue the "what's broken" is more important, but
for skimming logs and git archaeology, the gory details matter less and less.
E.g. when doing a series of "git blame", the details of each change along the
way are useless, the details only matter for the culprit.  Providing the "what
changed" makes it easy to quickly determine whether or not a commit might be of
interest.

Another benefit of stating "what's changing" first is that it's almost always
possible to state "what's changing" in a single sentence.  Conversely, all but
the most simple bugs require multiple sentences or paragraphs to fully describe
the problem.  If both the "what's changing" and "what's the bug" are super
short then the order doesn't matter.  But if one is shorter (almost always the
"what's changing), then covering the shorter one first is advantageous because
it's less of an inconvenience for readers/reviewers that have a strict ordering
preference.  E.g. having to skip one sentence to get to the context is less
painful than having to skip three paragraphs to get to "what's changing".

Fixes
~~~~~
If a change fixes a KVM/kernel bug, add a Fixes: tag even if the change doesn't
need to be backported to stable kernels, and even if the change fixes a bug in
an older release.

Conversely, if a fix does need to be backported, explicitly tag the patch with
"Cc: stable@vger.kernel" (though the email itself doesn't need to Cc: stable);
KVM x86 opts out of backporting Fixes: by default.  Some auto-selected patches
do get backported, but require explicit maintainer approval (search MANUALSEL).

Function References
~~~~~~~~~~~~~~~~~~~
When a function is mentioned in a comment, changelog, or shortlog (or anywhere
for that matter), use the format ``function_name()``.  The parentheses provide
context and disambiguate the reference.

Testing
-------
At a bare minimum, *all* patches in a series must build cleanly for KVM_INTEL=m
KVM_AMD=m, and KVM_WERROR=y.  Building every possible combination of Kconfigs
isn't feasible, but the more the merrier.  KVM_SMM, KVM_XEN, PROVE_LOCKING, and
X86_64 are particularly interesting knobs to turn.

Running KVM selftests and KVM-unit-tests is also mandatory (and stating the
obvious, the tests need to pass).  The only exception is for changes that have
negligible probability of affecting runtime behavior, e.g. patches that only
modify comments.  When possible and relevant, testing on both Intel and AMD is
strongly preferred.  Booting an actual VM is encouraged, but not mandatory.

For changes that touch KVM's shadow paging code, running with TDP (EPT/NPT)
disabled is mandatory.  For changes that affect common KVM MMU code, running
with TDP disabled is strongly encouraged.  For all other changes, if the code
being modified depends on and/or interacts with a module param, testing with
the relevant settings is mandatory.

Note, KVM selftests and KVM-unit-tests do have known failures.  If you suspect
a failure is not due to your changes, verify that the *exact same* failure
occurs with and without your changes.

Changes that touch reStructured Text documentation, i.e. .rst files, must build
htmldocs cleanly, i.e. with no new warnings or errors.

If you can't fully test a change, e.g. due to lack of hardware, clearly state
what level of testing you were able to do, e.g. in the cover letter.

New Features
~~~~~~~~~~~~
With one exception, new features *must* come with test coverage.  KVM specific
tests aren't strictly required, e.g. if coverage is provided by running a
sufficiently enabled guest VM, or by running a related kernel selftest in a VM,
but dedicated KVM tests are preferred in all cases.  Negative testcases in
particular are mandatory for enabling of new hardware features as error and
exception flows are rarely exercised simply by running a VM.

The only exception to this rule is if KVM is simply advertising support for a
feature via KVM_GET_SUPPORTED_CPUID, i.e. for instructions/features that KVM
can't prevent a guest from using and for which there is no true enabling.

Note, "new features" does not just mean "new hardware features"!  New features
that can't be well validated using existing KVM selftests and/or KVM-unit-tests
must come with tests.

Posting new feature development without tests to get early feedback is more
than welcome, but such submissions should be tagged RFC, and the cover letter
should clearly state what type of feedback is requested/expected.  Do not abuse
the RFC process; RFCs will typically not receive in-depth review.

Bug Fixes
~~~~~~~~~
Except for "obvious" found-by-inspection bugs, fixes must be accompanied by a
reproducer for the bug being fixed.  In many cases the reproducer is implicit,
e.g. for build errors and test failures, but it should still be clear to
readers what is broken and how to verify the fix.  Some leeway is given for
bugs that are found via non-public workloads/tests, but providing regression
tests for such bugs is strongly preferred.

In general, regression tests are preferred for any bug that is not trivial to
hit.  E.g. even if the bug was originally found by a fuzzer such as syzkaller,
a targeted regression test may be warranted if the bug requires hitting a
one-in-a-million type race condition.

Note, KVM bugs are rarely urgent *and* non-trivial to reproduce.  Ask yourself
if a bug is really truly the end of the world before posting a fix without a
reproducer.

Posting
-------

Links
~~~~~
Do not explicitly reference bug reports, prior versions of a patch/series, etc.
via ``In-Reply-To:`` headers.  Using ``In-Reply-To:`` becomes an unholy mess
for large series and/or when the version count gets high, and ``In-Reply-To:``
is useless for anyone that doesn't have the original message, e.g. if someone
wasn't Cc'd on the bug report or if the list of recipients changes between
versions.

To link to a bug report, previous version, or anything of interest, use lore
links.  For referencing previous version(s), generally speaking do not include
a Link: in the changelog as there is no need to record the history in git, i.e.
put the link in the cover letter or in the section git ignores.  Do provide a
formal Link: for bug reports and/or discussions that led to the patch.  The
context of why a change was made is highly valuable for future readers.

Git Base
~~~~~~~~
If you are using git version 2.9.0 or later (Googlers, this is all of you!),
use ``git format-patch`` with the ``--base`` flag to automatically include the
base tree information in the generated patches.

Note, ``--base=auto`` works as expected if and only if a branch's upstream is
set to the base topic branch, e.g. it will do the wrong thing if your upstream
is set to your personal repository for backup purposes.  An alternative "auto"
solution is to derive the names of your development branches based on their
KVM x86 topic, and feed that into ``--base``.  E.g. ``x86/pmu/my_branch_name``,
and then write a small wrapper to extract ``pmu`` from the current branch name
to yield ``--base=x/pmu``, where ``x`` is whatever name your repository uses to
track the KVM x86 remote.

Co-Posting Tests
~~~~~~~~~~~~~~~~
KVM selftests that are associated with KVM changes, e.g. regression tests for
bug fixes, should be posted along with the KVM changes as a single series.  The
standard kernel rules for bisection apply, i.e. KVM changes that result in test
failures should be ordered after the selftests updates, and vice versa, new
tests that fail due to KVM bugs should be ordered after the KVM fixes.

KVM-unit-tests should *always* be posted separately.  Tools, e.g. b4 am, don't
know that KVM-unit-tests is a separate repository and get confused when patches
in a series apply on different trees.  To tie KVM-unit-tests patches back to
KVM patches, first post the KVM changes and then provide a lore Link: to the
KVM patch/series in the KVM-unit-tests patch(es).

Notifications
-------------
When a patch/series is officially accepted, a notification email will be sent
in reply to the original posting (cover letter for multi-patch series).  The
notification will include the tree and topic branch, along with the SHA1s of
the commits of applied patches.

If a subset of patches is applied, this will be clearly stated in the
notification.  Unless stated otherwise, it's implied that any patches in the
series that were not accepted need more work and should be submitted in a new
version.

If for some reason a patch is dropped after officially being accepted, a reply
will be sent to the notification email explaining why the patch was dropped, as
well as the next steps.

SHA1 Stability
~~~~~~~~~~~~~~
SHA1s are not 100% guaranteed to be stable until they land in Linus' tree!  A
SHA1 is *usually* stable once a notification has been sent, but things happen.
In most cases, an update to the notification email be provided if an applied
patch's SHA1 changes.  However, in some scenarios, e.g. if all KVM x86 branches
need to be rebased, individual notifications will not be given.

Vulnerabilities
---------------
Bugs that can be exploited by the guest to attack the host (kernel or
userspace), or that can be exploited by a nested VM to *its* host (L2 attacking
L1), are of particular interest to KVM.  Please follow the protocol for
:ref:`securitybugs` if you suspect a bug can lead to an escape, data leak, etc.