Re: [PATCH 2/2] Documentation/process: Add a maintainer handbook for KVM x86

[Like Xu]

On 18/2/2023 6:54 am, Sean Christopherson wrote:
> Add a KVM x86 doc to the subsystem/maintainer handbook section to explain
> how KVM x86 (currently) operates as a sub-subsystem, and to soapbox on
> the rules and expectations for contributing to KVM x86.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   .../process/maintainer-handbooks.rst          |   1 +
>   Documentation/process/maintainer-kvm-x86.rst  | 347 ++++++++++++++++++
>   MAINTAINERS                                   |   1 +
>   3 files changed, 349 insertions(+)
>   create mode 100644 Documentation/process/maintainer-kvm-x86.rst
>
> diff --git a/Documentation/process/maintainer-handbooks.rst b/Documentation/process/maintainer-handbooks.rst
> index d783060b4cc6..d12cbbe2b7df 100644
> --- a/Documentation/process/maintainer-handbooks.rst
> +++ b/Documentation/process/maintainer-handbooks.rst
> @@ -17,3 +17,4 @@ Contents:
>   
>      maintainer-tip
>      maintainer-netdev
> +   maintainer-kvm-x86
> diff --git a/Documentation/process/maintainer-kvm-x86.rst b/Documentation/process/maintainer-kvm-x86.rst
> new file mode 100644
> index 000000000000..ac81a42a32a3
> --- /dev/null
> +++ b/Documentation/process/maintainer-kvm-x86.rst
> @@ -0,0 +1,347 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +KVM x86

KERNEL VIRTUAL MACHINE FOR X86 (KVM/x86)

> +=======
> +
> +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.
> +
> +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 cthulu merge on an as-needed basis, i.e. when a topic branch is updated.

s/cthulu/Cthulhu

> +As a result, force pushes to ``next`` are common.
> +
> +Lifecycle
> +~~~~~~~~~
> +Pull requests for the next release cycle are sent to the main KVM tree, one
> +for each KVM x86 topic branch.  If all goes well, the topic branches are rolled
> +into the main KVM pull request sent during Linus' merge window.  Pull requests
> +for KVM x86 branches are typically made the week before Linus' opening of the
> +merge window, e.g. the week following rc7 for "normal" releases.
> +
> +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.

Any urgent AND critical fixes are exempt. No?

> +
> +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 mainline, i.e. the current release, should be based on
> +``git://git.kernel.org/pub/scm/virt/kvm/kvm.git master``.
> +
> +Everything else should be based on a kvm-x86 topic branch.  If there is no
> +obvious fit, use ``misc``.  Unless a patch/series depends on and/or conflicts
> +with multiple topic branches, do not use ``next`` as a base.  Because ``next``
> +is force-pushed on a regular basis, depending on when others fetch ``next``,
> +they may or may not have the relevant objects in their local git tree.
> +
> +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.

priority in KVM x86 (including selftests).

> +
> +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 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.

ISE: Intel® Architecture Instruction Set Extensions and Future Features
PPR: Processor Programming Reference (PPR) for specific AMD Model

> +
> +Do not reference specific sections, tables, figures, etc. by number, especially
> +not in comments.  Instead, copy-paste the relevant snippet (if warranted), 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/consistent.
> +
> +Generally speaking, do not copy-paste SDM or APM snippets into comments.  With
> +few exceptions, KVM *must* honor architectural behavior, therefore it's implied
> +that KVM behavior is emulating SDM and/or APM behavior.

All undefined behaviors (if any) need to be clarified.

> +
> +Shortlog
> +~~~~~~~~
> +The preferred prefix format is ``KVM: <topic>:``, where ``<topic>`` is one of::
> +
> +  - x86
> +  - x86/mmu
> +  - x86/pmu
> +  - x86/xen

Any conflict w/ "KVM X86 Xen (KVM/Xen)" ? Then "KVM X86 HYPER-V (KVM/hyper-v)" ?

> +  - selftests > +  - SVM
> +  - nSVM
> +  - VMX
> +  - nVMX

emulator ? lapic ?

> +
> +**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.
> +
> +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.::

s/punctionation/punctuation

> +
> +    KVM: x86: Fix a null pointer dererence in function_xyz()

s/dererence/dereference

> +
> +not::
> +
> +    kvm: x86: fix a null pointer dererence 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.
> +
> +Do not use file names or complete file paths as the subject/shortlog prefix.
> +
> +Changelog
> +~~~~~~~~~
> +Write changelogs using imperative mood and avoid pronouns.  Lead with a short
> +blurb on what is changing, and then follow up with the context and background.
> +Note!  This order directly conflicts with the tip tree's preferred approach!

Emm, could this be considered/clarified as an incentive option ?

> +
> +Beyond personal preference, there are less subjective reasons for stating what
> +a patch does before diving into details.  First and foremost, what code is
> +actually being changed is 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".

I'm sure more than one kernel friends will be concerned about this requirement,
especially those who like to read novels in our git logs.

> +
> +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).  When possible and relevant, testing on both
> +Intel and AMD is strongly preferred.  Booting an actual VM is encouraged, but
> +not mandatory.

Testing L2 guest available features inside L1 is also encouraged.

> +
> +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.
> +
> +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.

Add an RFT (request for test) tag.

> +
> +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_SET_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.

must come with tests to ensure good code coverage.

> +
> +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.

tests or detailed reproduction sequence 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!),

Please do not mention specific developers or groups in this type of document.

> +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.

Keeping git-bisect is mandatory.

> +
> +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.

in a new version or a separate topic thread.

> +
> +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

L1, even L0 host),

> +:ref:`securitybugs` if you suspect a bug can lead to an escape, data leak, etc.
> +
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 6a47510d1592..13e67a8b4827 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -11436,6 +11436,7 @@ M:	Sean Christopherson <seanjc@xxxxxxxxxx>
>   M:	Paolo Bonzini <pbonzini@xxxxxxxxxx>
>   L:	kvm@xxxxxxxxxxxxxxx
>   S:	Supported
> +P:	Documentation/process/maintainer-kvm-x86.rst
>   T:	git git://git.kernel.org/pub/scm/virt/kvm/kvm.git
>   F:	arch/x86/include/asm/kvm*
>   F:	arch/x86/include/asm/svm.h