review-checklist.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3================================
  4Review checklist for kvm patches
  5================================
  6
  71.  The patch must follow Documentation/process/coding-style.rst and
  8    Documentation/process/submitting-patches.rst.
  9
 102.  Patches should be against kvm.git master or next branches.
 11
 123.  If the patch introduces or modifies a new userspace API:
 13    - the API must be documented in Documentation/virt/kvm/api.rst
 14    - the API must be discoverable using KVM_CHECK_EXTENSION
 15
 164.  New state must include support for save/restore.
 17
 185.  New features must default to off (userspace should explicitly request them).
 19    Performance improvements can and should default to on.
 20
 216.  New cpu features should be exposed via KVM_GET_SUPPORTED_CPUID2,
 22    or its equivalent for non-x86 architectures
 23
 247.  The feature should be testable (see below).
 25
 268.  Changes should be vendor neutral when possible.  Changes to common code
 27    are better than duplicating changes to vendor code.
 28
 299.  Similarly, prefer changes to arch independent code than to arch dependent
 30    code.
 31
 3210. User/kernel interfaces and guest/host interfaces must be 64-bit clean
 33    (all variables and sizes naturally aligned on 64-bit; use specific types
 34    only - u64 rather than ulong).
 35
 3611. New guest visible features must either be documented in a hardware manual
 37    or be accompanied by documentation.
 38
 39Testing of KVM code
 40-------------------
 41
 42All features contributed to KVM, and in many cases bugfixes too, should be
 43accompanied by some kind of tests and/or enablement in open source guests
 44and VMMs.  KVM is covered by multiple test suites:
 45
 46*Selftests*
 47  These are low level tests that allow granular testing of kernel APIs.
 48  This includes API failure scenarios, invoking APIs after specific
 49  guest instructions, and testing multiple calls to ``KVM_CREATE_VM``
 50  within a single test.  They are included in the kernel tree at
 51  ``tools/testing/selftests/kvm``.
 52
 53``kvm-unit-tests``
 54  A collection of small guests that test CPU and emulated device features
 55  from a guest's perspective.  They run under QEMU or ``kvmtool``, and
 56  are generally not KVM-specific: they can be run with any accelerator
 57  that QEMU support or even on bare metal, making it possible to compare
 58  behavior across hypervisors and processor families.
 59
 60Functional test suites
 61  Various sets of functional tests exist, such as QEMU's ``tests/functional``
 62  suite and `avocado-vt <https://avocado-vt.readthedocs.io/en/latest/>`__.
 63  These typically involve running a full operating system in a virtual
 64  machine.
 65
 66The best testing approach depends on the feature's complexity and
 67operation. Here are some examples and guidelines:
 68
 69New instructions (no new registers or APIs)
 70  The corresponding CPU features (if applicable) should be made available
 71  in QEMU.  If the instructions require emulation support or other code in
 72  KVM, it is worth adding coverage to ``kvm-unit-tests`` or selftests;
 73  the latter can be a better choice if the instructions relate to an API
 74  that already has good selftest coverage.
 75
 76New hardware features (new registers, no new APIs)
 77  These should be tested via ``kvm-unit-tests``; this more or less implies
 78  supporting them in QEMU and/or ``kvmtool``.  In some cases selftests
 79  can be used instead, similar to the previous case, or specifically to
 80  test corner cases in guest state save/restore.
 81
 82Bug fixes and performance improvements
 83  These usually do not introduce new APIs, but it's worth sharing
 84  any benchmarks and tests that will validate your contribution,
 85  ideally in the form of regression tests.  Tests and benchmarks
 86  can be included in either ``kvm-unit-tests`` or selftests, depending
 87  on the specifics of your change.  Selftests are especially useful for
 88  regression tests because they are included directly in Linux's tree.
 89
 90Large scale internal changes
 91  While it's difficult to provide a single policy, you should ensure that
 92  the changed code is covered by either ``kvm-unit-tests`` or selftests.
 93  In some cases the affected code is run for any guests and functional
 94  tests suffice.  Explain your testing process in the cover letter,
 95  as that can help identify gaps in existing test suites.
 96
 97New APIs
 98  It is important to demonstrate your use case.  This can be as simple as
 99  explaining that the feature is already in use on bare metal, or it can be
100  a proof-of-concept implementation in userspace.  The latter need not be
101  open source, though that is of course preferable for easier testing.
102  Selftests should test corner cases of the APIs, and should also cover
103  basic host and guest operation if no open source VMM uses the feature.
104
105Bigger features, usually spanning host and guest
106  These should be supported by Linux guests, with limited exceptions for
107  Hyper-V features that are testable on Windows guests.  It is strongly
108  suggested that the feature be usable with an open source host VMM, such
109  as at least one of QEMU or crosvm, and guest firmware.  Selftests should
110  test at least API error cases.  Guest operation can be covered by
111  either selftests of ``kvm-unit-tests`` (this is especially important for
112  paravirtualized and Windows-only features).  Strong selftest coverage
113  can also be a replacement for implementation in an open source VMM,
114  but this is generally not recommended.
115
116Following the above suggestions for testing in selftests and
117``kvm-unit-tests`` will make it easier for the maintainers to review
118and accept your code.  In fact, even before you contribute your changes
119upstream it will make it easier for you to develop for KVM.
120
121Of course, the KVM maintainers reserve the right to require more tests,
122though they may also waive the requirement from time to time.