bpf, doc: add faq about bpf development process

In the same spirit of netdev FAQ, start a BPF FAQ as a collection
of expectations and/or workflow details in the context of BPF patch
processing.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/bpf_devel_QA.txt

+This document provides information for the BPF subsystem about various
+workflows related to reporting bugs, submitting patches, and queueing
+patches for stable kernels.
+
+For general information about submitting patches, please refer to
+Documentation/process/. This document only describes additional specifics
+related to BPF.
+
+Reporting bugs:
+---------------
+
+Q: How do I report bugs for BPF kernel code?
+
+A: Since all BPF kernel development as well as bpftool and iproute2 BPF
+   loader development happens through the netdev kernel mailing list,
+   please report any found issues around BPF to the following mailing
+   list:
+
+     netdev@vger.kernel.org
+
+   This may also include issues related to XDP, BPF tracing, etc.
+
+   Given netdev has a high volume of traffic, please also add the BPF
+   maintainers to Cc (from kernel MAINTAINERS file):
+
+     Alexei Starovoitov <ast@kernel.org>
+     Daniel Borkmann <daniel@iogearbox.net>
+
+   In case a buggy commit has already been identified, make sure to keep
+   the actual commit authors in Cc as well for the report. They can
+   typically be identified through the kernel's git tree.
+
+   Please do *not* report BPF issues to bugzilla.kernel.org since it
+   is a guarantee that the reported issue will be overlooked.
+
+Submitting patches:
+-------------------
+
+Q: To which mailing list do I need to submit my BPF patches?
+
+A: Please submit your BPF patches to the netdev kernel mailing list:
+
+     netdev@vger.kernel.org
+
+   Historically, BPF came out of networking and has always been maintained
+   by the kernel networking community. Although these days BPF touches
+   many other subsystems as well, the patches are still routed mainly
+   through the networking community.
+
+   In case your patch has changes in various different subsystems (e.g.
+   tracing, security, etc), make sure to Cc the related kernel mailing
+   lists and maintainers from there as well, so they are able to review
+   the changes and provide their Acked-by's to the patches.
+
+Q: Where can I find patches currently under discussion for BPF subsystem?
+
+A: All patches that are Cc'ed to netdev are queued for review under netdev
+   patchwork project:
+
+     http://patchwork.ozlabs.org/project/netdev/list/
+
+   Those patches which target BPF, are assigned to a 'bpf' delegate for
+   further processing from BPF maintainers. The current queue with
+   patches under review can be found at:
+
+     https://patchwork.ozlabs.org/project/netdev/list/?delegate=77147
+
+   Once the patches have been reviewed by the BPF community as a whole
+   and approved by the BPF maintainers, their status in patchwork will be
+   changed to 'Accepted' and the submitter will be notified by mail. This
+   means that the patches look good from a BPF perspective and have been
+   applied to one of the two BPF kernel trees.
+
+   In case feedback from the community requires a respin of the patches,
+   their status in patchwork will be set to 'Changes Requested', and purged
+   from the current review queue. Likewise for cases where patches would
+   get rejected or are not applicable to the BPF trees (but assigned to
+   the 'bpf' delegate).
+
+Q: How do the changes make their way into Linux?
+
+A: There are two BPF kernel trees (git repositories). Once patches have
+   been accepted by the BPF maintainers, they will be applied to one
+   of the two BPF trees:
+
+     https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/
+     https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/
+
+   The bpf tree itself is for fixes only, whereas bpf-next for features,
+   cleanups or other kind of improvements ("next-like" content). This is
+   analogous to net and net-next trees for networking. Both bpf and
+   bpf-next will only have a master branch in order to simplify against
+   which branch patches should get rebased to.
+
+   Accumulated BPF patches in the bpf tree will regularly get pulled
+   into the net kernel tree. Likewise, accumulated BPF patches accepted
+   into the bpf-next tree will make their way into net-next tree. net and
+   net-next are both run by David S. Miller. From there, they will go
+   into the kernel mainline tree run by Linus Torvalds. To read up on the
+   process of net and net-next being merged into the mainline tree, see
+   the netdev FAQ under:
+
+     Documentation/networking/netdev-FAQ.txt
+
+   Occasionally, to prevent merge conflicts, we might send pull requests
+   to other trees (e.g. tracing) with a small subset of the patches, but
+   net and net-next are always the main trees targeted for integration.
+
+   The pull requests will contain a high-level summary of the accumulated
+   patches and can be searched on netdev kernel mailing list through the
+   following subject lines (yyyy-mm-dd is the date of the pull request):
+
+     pull-request: bpf yyyy-mm-dd
+     pull-request: bpf-next yyyy-mm-dd
+
+Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be
+   applied to?
+
+A: The process is the very same as described in the netdev FAQ, so
+   please read up on it. The subject line must indicate whether the
+   patch is a fix or rather "next-like" content in order to let the
+   maintainers know whether it is targeted at bpf or bpf-next.
+
+   For fixes eventually landing in bpf -> net tree, the subject must
+   look like:
+
+     git format-patch --subject-prefix='PATCH bpf' start..finish
+
+   For features/improvements/etc that should eventually land in
+   bpf-next -> net-next, the subject must look like:
+
+     git format-patch --subject-prefix='PATCH bpf-next' start..finish
+
+   If unsure whether the patch or patch series should go into bpf
+   or net directly, or bpf-next or net-next directly, it is not a
+   problem either if the subject line says net or net-next as target.
+   It is eventually up to the maintainers to do the delegation of
+   the patches.
+
+   If it is clear that patches should go into bpf or bpf-next tree,
+   please make sure to rebase the patches against those trees in
+   order to reduce potential conflicts.
+
+   In case the patch or patch series has to be reworked and sent out
+   again in a second or later revision, it is also required to add a
+   version number (v2, v3, ...) into the subject prefix:
+
+     git format-patch --subject-prefix='PATCH net-next v2' start..finish
+
+   When changes have been requested to the patch series, always send the
+   whole patch series again with the feedback incorporated (never send
+   individual diffs on top of the old series).
+
+Q: What does it mean when a patch gets applied to bpf or bpf-next tree?
+
+A: It means that the patch looks good for mainline inclusion from
+   a BPF point of view.
+
+   Be aware that this is not a final verdict that the patch will
+   automatically get accepted into net or net-next trees eventually:
+
+   On the netdev kernel mailing list reviews can come in at any point
+   in time. If discussions around a patch conclude that they cannot
+   get included as-is, we will either apply a follow-up fix or drop
+   them from the trees entirely. Therefore, we also reserve to rebase
+   the trees when deemed necessary. After all, the purpose of the tree
+   is to i) accumulate and stage BPF patches for integration into trees
+   like net and net-next, and ii) run extensive BPF test suite and
+   workloads on the patches before they make their way any further.
+
+   Once the BPF pull request was accepted by David S. Miller, then
+   the patches end up in net or net-next tree, respectively, and
+   make their way from there further into mainline. Again, see the
+   netdev FAQ for additional information e.g. on how often they are
+   merged to mainline.
+
+Q: How long do I need to wait for feedback on my BPF patches?
+
+A: We try to keep the latency low. The usual time to feedback will
+   be around 2 or 3 business days. It may vary depending on the
+   complexity of changes and current patch load.
+
+Q: How often do you send pull requests to major kernel trees like
+   net or net-next?
+
+A: Pull requests will be sent out rather often in order to not
+   accumulate too many patches in bpf or bpf-next.
+
+   As a rule of thumb, expect pull requests for each tree regularly
+   at the end of the week. In some cases pull requests could additionally
+   come also in the middle of the week depending on the current patch
+   load or urgency.
+
+Q: Are patches applied to bpf-next when the merge window is open?
+
+A: For the time when the merge window is open, bpf-next will not be
+   processed. This is roughly analogous to net-next patch processing,
+   so feel free to read up on the netdev FAQ about further details.
+
+   During those two weeks of merge window, we might ask you to resend
+   your patch series once bpf-next is open again. Once Linus released
+   a v*-rc1 after the merge window, we continue processing of bpf-next.
+
+   For non-subscribers to kernel mailing lists, there is also a status
+   page run by David S. Miller on net-next that provides guidance:
+
+     http://vger.kernel.org/~davem/net-next.html
+
+Q: I made a BPF verifier change, do I need to add test cases for
+   BPF kernel selftests?
+
+A: If the patch has changes to the behavior of the verifier, then yes,
+   it is absolutely necessary to add test cases to the BPF kernel
+   selftests suite. If they are not present and we think they are
+   needed, then we might ask for them before accepting any changes.
+
+   In particular, test_verifier.c is tracking a high number of BPF test
+   cases, including a lot of corner cases that LLVM BPF back end may
+   generate out of the restricted C code. Thus, adding test cases is
+   absolutely crucial to make sure future changes do not accidentally
+   affect prior use-cases. Thus, treat those test cases as: verifier
+   behavior that is not tracked in test_verifier.c could potentially
+   be subject to change.
+
+Q: When should I add code to samples/bpf/ and when to BPF kernel
+   selftests?
+
+A: In general, we prefer additions to BPF kernel selftests rather than
+   samples/bpf/. The rationale is very simple: kernel selftests are
+   regularly run by various bots to test for kernel regressions.
+
+   The more test cases we add to BPF selftests, the better the coverage
+   and the less likely it is that those could accidentally break. It is
+   not that BPF kernel selftests cannot demo how a specific feature can
+   be used.
+
+   That said, samples/bpf/ may be a good place for people to get started,
+   so it might be advisable that simple demos of features could go into
+   samples/bpf/, but advanced functional and corner-case testing rather
+   into kernel selftests.
+
+   If your sample looks like a test case, then go for BPF kernel selftests
+   instead!
+
+Q: When should I add code to the bpftool?
+
+A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
+   a central user space tool for debugging and introspection of BPF programs
+   and maps that are active in the kernel. If UAPI changes related to BPF
+   enable for dumping additional information of programs or maps, then
+   bpftool should be extended as well to support dumping them.
+
+Q: When should I add code to iproute2's BPF loader?
+
+A: For UAPI changes related to the XDP or tc layer (e.g. cls_bpf), the
+   convention is that those control-path related changes are added to
+   iproute2's BPF loader as well from user space side. This is not only
+   useful to have UAPI changes properly designed to be usable, but also
+   to make those changes available to a wider user base of major
+   downstream distributions.
+
+Q: Do you accept patches as well for iproute2's BPF loader?
+
+A: Patches for the iproute2's BPF loader have to be sent to:
+
+     netdev@vger.kernel.org
+
+   While those patches are not processed by the BPF kernel maintainers,
+   please keep them in Cc as well, so they can be reviewed.
+
+   The official git repository for iproute2 is run by Stephen Hemminger
+   and can be found at:
+
+     https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/
+
+   The patches need to have a subject prefix of '[PATCH iproute2 master]'
+   or '[PATCH iproute2 net-next]'. 'master' or 'net-next' describes the
+   target branch where the patch should be applied to. Meaning, if kernel
+   changes went into the net-next kernel tree, then the related iproute2
+   changes need to go into the iproute2 net-next branch, otherwise they
+   can be targeted at master branch. The iproute2 net-next branch will get
+   merged into the master branch after the current iproute2 version from
+   master has been released.
+
+   Like BPF, the patches end up in patchwork under the netdev project and
+   are delegated to 'shemminger' for further processing:
+
+     http://patchwork.ozlabs.org/project/netdev/list/?delegate=389
+
+Q: What is the minimum requirement before I submit my BPF patches?
+
+A: When submitting patches, always take the time and properly test your
+   patches *prior* to submission. Never rush them! If maintainers find
+   that your patches have not been properly tested, it is a good way to
+   get them grumpy. Testing patch submissions is a hard requirement!
+
+   Note, fixes that go to bpf tree *must* have a Fixes: tag included. The
+   same applies to fixes that target bpf-next, where the affected commit
+   is in net-next (or in some cases bpf-next). The Fixes: tag is crucial
+   in order to identify follow-up commits and tremendously helps for people
+   having to do backporting, so it is a must have!
+
+   We also don't accept patches with an empty commit message. Take your
+   time and properly write up a high quality commit message, it is
+   essential!
+
+   Think about it this way: other developers looking at your code a month
+   from now need to understand *why* a certain change has been done that
+   way, and whether there have been flaws in the analysis or assumptions
+   that the original author did. Thus providing a proper rationale and
+   describing the use-case for the changes is a must.
+
+   Patch submissions with >1 patch must have a cover letter which includes
+   a high level description of the series. This high level summary will
+   then be placed into the merge commit by the BPF maintainers such that
+   it is also accessible from the git log for future reference.
+
+Q: What do I need to consider when adding a new instruction or feature
+   that would require BPF JIT and/or LLVM integration as well?
+
+A: We try hard to keep all BPF JITs up to date such that the same user
+   experience can be guaranteed when running BPF programs on different
+   architectures without having the program punt to the less efficient
+   interpreter in case the in-kernel BPF JIT is enabled.
+
+   If you are unable to implement or test the required JIT changes for
+   certain architectures, please work together with the related BPF JIT
+   developers in order to get the feature implemented in a timely manner.
+   Please refer to the git log (arch/*/net/) to locate the necessary
+   people for helping out.
+
+   Also always make sure to add BPF test cases (e.g. test_bpf.c and
+   test_verifier.c) for new instructions, so that they can receive
+   broad test coverage and help run-time testing the various BPF JITs.
+
+   In case of new BPF instructions, once the changes have been accepted
+   into the Linux kernel, please implement support into LLVM's BPF back
+   end. See LLVM section below for further information.
+
+Stable submission:
+------------------
+
+Q: I need a specific BPF commit in stable kernels. What should I do?
+
+A: In case you need a specific fix in stable kernels, first check whether
+   the commit has already been applied in the related linux-*.y branches:
+
+     https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/
+
+   If not the case, then drop an email to the BPF maintainers with the
+   netdev kernel mailing list in Cc and ask for the fix to be queued up:
+
+     netdev@vger.kernel.org
+
+   The process in general is the same as on netdev itself, see also the
+   netdev FAQ document.
+
+Q: Do you also backport to kernels not currently maintained as stable?
+
+A: No. If you need a specific BPF commit in kernels that are currently not
+   maintained by the stable maintainers, then you are on your own.
+
+   The current stable and longterm stable kernels are all listed here:
+
+     https://www.kernel.org/
+
+Q: The BPF patch I am about to submit needs to go to stable as well. What
+   should I do?
+
+A: The same rules apply as with netdev patch submissions in general, see
+   netdev FAQ under:
+
+     Documentation/networking/netdev-FAQ.txt
+
+   Never add "Cc: stable@vger.kernel.org" to the patch description, but
+   ask the BPF maintainers to queue the patches instead. This can be done
+   with a note, for example, under the "---" part of the patch which does
+   not go into the git log. Alternatively, this can be done as a simple
+   request by mail instead.
+
+Q: Where do I find currently queued BPF patches that will be submitted
+   to stable?
+
+A: Once patches that fix critical bugs got applied into the bpf tree, they
+   are queued up for stable submission under:
+
+     http://patchwork.ozlabs.org/bundle/bpf/stable/?state=*
+
+   They will be on hold there at minimum until the related commit made its
+   way into the mainline kernel tree.
+
+   After having been under broader exposure, the queued patches will be
+   submitted by the BPF maintainers to the stable maintainers.
+
+Testing patches:
+----------------
+
+Q: Which BPF kernel selftests version should I run my kernel against?
+
+A: If you run a kernel xyz, then always run the BPF kernel selftests from
+   that kernel xyz as well. Do not expect that the BPF selftest from the
+   latest mainline tree will pass all the time.
+
+   In particular, test_bpf.c and test_verifier.c have a large number of
+   test cases and are constantly updated with new BPF test sequences, or
+   existing ones are adapted to verifier changes e.g. due to verifier
+   becoming smarter and being able to better track certain things.
+
+LLVM:
+-----
+
+Q: Where do I find LLVM with BPF support?
+
+A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.
+
+   All major distributions these days ship LLVM with BPF back end enabled,
+   so for the majority of use-cases it is not required to compile LLVM by
+   hand anymore, just install the distribution provided package.
+
+   LLVM's static compiler lists the supported targets through 'llc --version',
+   make sure BPF targets are listed. Example:
+
+     $ llc --version
+     LLVM (http://llvm.org/):
+       LLVM version 6.0.0svn
+       Optimized build.
+       Default target: x86_64-unknown-linux-gnu
+       Host CPU: skylake
+
+       Registered Targets:
+         bpf    - BPF (host endian)
+         bpfeb  - BPF (big endian)
+         bpfel  - BPF (little endian)
+         x86    - 32-bit X86: Pentium-Pro and above
+         x86-64 - 64-bit X86: EM64T and AMD64
+
+   For developers in order to utilize the latest features added to LLVM's
+   BPF back end, it is advisable to run the latest LLVM releases. Support
+   for new BPF kernel features such as additions to the BPF instruction
+   set are often developed together.
+
+   All LLVM releases can be found at: http://releases.llvm.org/
+
+Q: Got it, so how do I build LLVM manually anyway?
+
+A: You need cmake and gcc-c++ as build requisites for LLVM. Once you have
+   that set up, proceed with building the latest LLVM and clang version
+   from the git repositories:
+
+     $ git clone http://llvm.org/git/llvm.git
+     $ cd llvm/tools
+     $ git clone --depth 1 http://llvm.org/git/clang.git
+     $ cd ..; mkdir build; cd build
+     $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
+                -DBUILD_SHARED_LIBS=OFF           \
+                -DCMAKE_BUILD_TYPE=Release        \
+                -DLLVM_BUILD_RUNTIME=OFF
+     $ make -j $(getconf _NPROCESSORS_ONLN)
+
+   The built binaries can then be found in the build/bin/ directory, where
+   you can point the PATH variable to.
+
+Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code
+   generation back end or about LLVM generated code that the verifier
+   refuses to accept?
+
+A: Yes, please do! LLVM's BPF back end is a key piece of the whole BPF
+   infrastructure and it ties deeply into verification of programs from the
+   kernel side. Therefore, any issues on either side need to be investigated
+   and fixed whenever necessary.
+
+   Therefore, please make sure to bring them up at netdev kernel mailing
+   list and Cc BPF maintainers for LLVM and kernel bits:
+
+     Yonghong Song <yhs@fb.com>
+     Alexei Starovoitov <ast@kernel.org>
+     Daniel Borkmann <daniel@iogearbox.net>
+
+   LLVM also has an issue tracker where BPF related bugs can be found:
+
+     https://bugs.llvm.org/buglist.cgi?quicksearch=bpf
+
+   However, it is better to reach out through mailing lists with having
+   maintainers in Cc.
+
+Q: I have added a new BPF instruction to the kernel, how can I integrate
+   it into LLVM?
+
+A: LLVM has a -mcpu selector for the BPF back end in order to allow the
+   selection of BPF instruction set extensions. By default the 'generic'
+   processor target is used, which is the base instruction set (v1) of BPF.
+
+   LLVM has an option to select -mcpu=probe where it will probe the host
+   kernel for supported BPF instruction set extensions and selects the
+   optimal set automatically.
+
+   For cross-compilation, a specific version can be select manually as well.
+
+     $ llc -march bpf -mcpu=help
+     Available CPUs for this target:
+
+       generic - Select the generic processor.
+       probe   - Select the probe processor.
+       v1      - Select the v1 processor.
+       v2      - Select the v2 processor.
+     [...]
+
+   Newly added BPF instructions to the Linux kernel need to follow the same
+   scheme, bump the instruction set version and implement probing for the
+   extensions such that -mcpu=probe users can benefit from the optimization
+   transparently when upgrading their kernels.
+
+   If you are unable to implement support for the newly added BPF instruction
+   please reach out to BPF developers for help.
+
+   By the way, the BPF kernel selftests run with -mcpu=probe for better
+   test coverage.
+
+Happy BPF hacking!