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diff --git a/Documentation/process/maintainer-kvm-x86.rst b/Documentation/process/maintainer-kvm-x86.rst new file mode 100644 index 000000000000..9183bd449762 --- /dev/null +++ b/Documentation/process/maintainer-kvm-x86.rst @@ -0,0 +1,390 @@ +.. 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. + |