On Fri, Feb 17, 2023, 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 > +======= > + > +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. > +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. > + > +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. > + > +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. What is the 'reverse fir tree'? Maybe, "Reverse Christmas Tree" is better to understand. > + > +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, 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. > + > +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. > + > +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 dererence in function_xyz() > + > +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! > + > +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". > + > +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. > + > +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. > + > +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. > + > +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. > + > +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. > + > 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 > -- > 2.39.2.637.g21b0678d19-goog >
