linux/Documentation/bpf/bpf_devel_QA.rst
<<
>>
Prefs
   1=================================
   2HOWTO interact with BPF subsystem
   3=================================
   4
   5This document provides information for the BPF subsystem about various
   6workflows related to reporting bugs, submitting patches, and queueing
   7patches for stable kernels.
   8
   9For general information about submitting patches, please refer to
  10`Documentation/process/`_. This document only describes additional specifics
  11related to BPF.
  12
  13.. contents::
  14    :local:
  15    :depth: 2
  16
  17Reporting bugs
  18==============
  19
  20Q: How do I report bugs for BPF kernel code?
  21--------------------------------------------
  22A: Since all BPF kernel development as well as bpftool and iproute2 BPF
  23loader development happens through the bpf kernel mailing list,
  24please report any found issues around BPF to the following mailing
  25list:
  26
  27 bpf@vger.kernel.org
  28
  29This may also include issues related to XDP, BPF tracing, etc.
  30
  31Given netdev has a high volume of traffic, please also add the BPF
  32maintainers to Cc (from kernel ``MAINTAINERS`` file):
  33
  34* Alexei Starovoitov <ast@kernel.org>
  35* Daniel Borkmann <daniel@iogearbox.net>
  36
  37In case a buggy commit has already been identified, make sure to keep
  38the actual commit authors in Cc as well for the report. They can
  39typically be identified through the kernel's git tree.
  40
  41**Please do NOT report BPF issues to bugzilla.kernel.org since it
  42is a guarantee that the reported issue will be overlooked.**
  43
  44Submitting patches
  45==================
  46
  47Q: To which mailing list do I need to submit my BPF patches?
  48------------------------------------------------------------
  49A: Please submit your BPF patches to the bpf kernel mailing list:
  50
  51 bpf@vger.kernel.org
  52
  53In case your patch has changes in various different subsystems (e.g.
  54networking, tracing, security, etc), make sure to Cc the related kernel mailing
  55lists and maintainers from there as well, so they are able to review
  56the changes and provide their Acked-by's to the patches.
  57
  58Q: Where can I find patches currently under discussion for BPF subsystem?
  59-------------------------------------------------------------------------
  60A: All patches that are Cc'ed to netdev are queued for review under netdev
  61patchwork project:
  62
  63  https://patchwork.kernel.org/project/netdevbpf/list/
  64
  65Those patches which target BPF, are assigned to a 'bpf' delegate for
  66further processing from BPF maintainers. The current queue with
  67patches under review can be found at:
  68
  69  https://patchwork.kernel.org/project/netdevbpf/list/?delegate=121173
  70
  71Once the patches have been reviewed by the BPF community as a whole
  72and approved by the BPF maintainers, their status in patchwork will be
  73changed to 'Accepted' and the submitter will be notified by mail. This
  74means that the patches look good from a BPF perspective and have been
  75applied to one of the two BPF kernel trees.
  76
  77In case feedback from the community requires a respin of the patches,
  78their status in patchwork will be set to 'Changes Requested', and purged
  79from the current review queue. Likewise for cases where patches would
  80get rejected or are not applicable to the BPF trees (but assigned to
  81the 'bpf' delegate).
  82
  83Q: How do the changes make their way into Linux?
  84------------------------------------------------
  85A: There are two BPF kernel trees (git repositories). Once patches have
  86been accepted by the BPF maintainers, they will be applied to one
  87of the two BPF trees:
  88
  89 * https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/
  90 * https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/
  91
  92The bpf tree itself is for fixes only, whereas bpf-next for features,
  93cleanups or other kind of improvements ("next-like" content). This is
  94analogous to net and net-next trees for networking. Both bpf and
  95bpf-next will only have a master branch in order to simplify against
  96which branch patches should get rebased to.
  97
  98Accumulated BPF patches in the bpf tree will regularly get pulled
  99into the net kernel tree. Likewise, accumulated BPF patches accepted
 100into the bpf-next tree will make their way into net-next tree. net and
 101net-next are both run by David S. Miller. From there, they will go
 102into the kernel mainline tree run by Linus Torvalds. To read up on the
 103process of net and net-next being merged into the mainline tree, see
 104the :ref:`netdev-FAQ`
 105
 106
 107
 108Occasionally, to prevent merge conflicts, we might send pull requests
 109to other trees (e.g. tracing) with a small subset of the patches, but
 110net and net-next are always the main trees targeted for integration.
 111
 112The pull requests will contain a high-level summary of the accumulated
 113patches and can be searched on netdev kernel mailing list through the
 114following subject lines (``yyyy-mm-dd`` is the date of the pull
 115request)::
 116
 117  pull-request: bpf yyyy-mm-dd
 118  pull-request: bpf-next yyyy-mm-dd
 119
 120Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be applied to?
 121---------------------------------------------------------------------------------
 122
 123A: The process is the very same as described in the :ref:`netdev-FAQ`,
 124so please read up on it. The subject line must indicate whether the
 125patch is a fix or rather "next-like" content in order to let the
 126maintainers know whether it is targeted at bpf or bpf-next.
 127
 128For fixes eventually landing in bpf -> net tree, the subject must
 129look like::
 130
 131  git format-patch --subject-prefix='PATCH bpf' start..finish
 132
 133For features/improvements/etc that should eventually land in
 134bpf-next -> net-next, the subject must look like::
 135
 136  git format-patch --subject-prefix='PATCH bpf-next' start..finish
 137
 138If unsure whether the patch or patch series should go into bpf
 139or net directly, or bpf-next or net-next directly, it is not a
 140problem either if the subject line says net or net-next as target.
 141It is eventually up to the maintainers to do the delegation of
 142the patches.
 143
 144If it is clear that patches should go into bpf or bpf-next tree,
 145please make sure to rebase the patches against those trees in
 146order to reduce potential conflicts.
 147
 148In case the patch or patch series has to be reworked and sent out
 149again in a second or later revision, it is also required to add a
 150version number (``v2``, ``v3``, ...) into the subject prefix::
 151
 152  git format-patch --subject-prefix='PATCH bpf-next v2' start..finish
 153
 154When changes have been requested to the patch series, always send the
 155whole patch series again with the feedback incorporated (never send
 156individual diffs on top of the old series).
 157
 158Q: What does it mean when a patch gets applied to bpf or bpf-next tree?
 159-----------------------------------------------------------------------
 160A: It means that the patch looks good for mainline inclusion from
 161a BPF point of view.
 162
 163Be aware that this is not a final verdict that the patch will
 164automatically get accepted into net or net-next trees eventually:
 165
 166On the bpf kernel mailing list reviews can come in at any point
 167in time. If discussions around a patch conclude that they cannot
 168get included as-is, we will either apply a follow-up fix or drop
 169them from the trees entirely. Therefore, we also reserve to rebase
 170the trees when deemed necessary. After all, the purpose of the tree
 171is to:
 172
 173i) accumulate and stage BPF patches for integration into trees
 174   like net and net-next, and
 175
 176ii) run extensive BPF test suite and
 177    workloads on the patches before they make their way any further.
 178
 179Once the BPF pull request was accepted by David S. Miller, then
 180the patches end up in net or net-next tree, respectively, and
 181make their way from there further into mainline. Again, see the
 182:ref:`netdev-FAQ` for additional information e.g. on how often they are
 183merged to mainline.
 184
 185Q: How long do I need to wait for feedback on my BPF patches?
 186-------------------------------------------------------------
 187A: We try to keep the latency low. The usual time to feedback will
 188be around 2 or 3 business days. It may vary depending on the
 189complexity of changes and current patch load.
 190
 191Q: How often do you send pull requests to major kernel trees like net or net-next?
 192----------------------------------------------------------------------------------
 193
 194A: Pull requests will be sent out rather often in order to not
 195accumulate too many patches in bpf or bpf-next.
 196
 197As a rule of thumb, expect pull requests for each tree regularly
 198at the end of the week. In some cases pull requests could additionally
 199come also in the middle of the week depending on the current patch
 200load or urgency.
 201
 202Q: Are patches applied to bpf-next when the merge window is open?
 203-----------------------------------------------------------------
 204A: For the time when the merge window is open, bpf-next will not be
 205processed. This is roughly analogous to net-next patch processing,
 206so feel free to read up on the :ref:`netdev-FAQ` about further details.
 207
 208During those two weeks of merge window, we might ask you to resend
 209your patch series once bpf-next is open again. Once Linus released
 210a ``v*-rc1`` after the merge window, we continue processing of bpf-next.
 211
 212For non-subscribers to kernel mailing lists, there is also a status
 213page run by David S. Miller on net-next that provides guidance:
 214
 215  http://vger.kernel.org/~davem/net-next.html
 216
 217Q: Verifier changes and test cases
 218----------------------------------
 219Q: I made a BPF verifier change, do I need to add test cases for
 220BPF kernel selftests_?
 221
 222A: If the patch has changes to the behavior of the verifier, then yes,
 223it is absolutely necessary to add test cases to the BPF kernel
 224selftests_ suite. If they are not present and we think they are
 225needed, then we might ask for them before accepting any changes.
 226
 227In particular, test_verifier.c is tracking a high number of BPF test
 228cases, including a lot of corner cases that LLVM BPF back end may
 229generate out of the restricted C code. Thus, adding test cases is
 230absolutely crucial to make sure future changes do not accidentally
 231affect prior use-cases. Thus, treat those test cases as: verifier
 232behavior that is not tracked in test_verifier.c could potentially
 233be subject to change.
 234
 235Q: samples/bpf preference vs selftests?
 236---------------------------------------
 237Q: When should I add code to ``samples/bpf/`` and when to BPF kernel
 238selftests_?
 239
 240A: In general, we prefer additions to BPF kernel selftests_ rather than
 241``samples/bpf/``. The rationale is very simple: kernel selftests are
 242regularly run by various bots to test for kernel regressions.
 243
 244The more test cases we add to BPF selftests, the better the coverage
 245and the less likely it is that those could accidentally break. It is
 246not that BPF kernel selftests cannot demo how a specific feature can
 247be used.
 248
 249That said, ``samples/bpf/`` may be a good place for people to get started,
 250so it might be advisable that simple demos of features could go into
 251``samples/bpf/``, but advanced functional and corner-case testing rather
 252into kernel selftests.
 253
 254If your sample looks like a test case, then go for BPF kernel selftests
 255instead!
 256
 257Q: When should I add code to the bpftool?
 258-----------------------------------------
 259A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
 260a central user space tool for debugging and introspection of BPF programs
 261and maps that are active in the kernel. If UAPI changes related to BPF
 262enable for dumping additional information of programs or maps, then
 263bpftool should be extended as well to support dumping them.
 264
 265Q: When should I add code to iproute2's BPF loader?
 266---------------------------------------------------
 267A: For UAPI changes related to the XDP or tc layer (e.g. ``cls_bpf``),
 268the convention is that those control-path related changes are added to
 269iproute2's BPF loader as well from user space side. This is not only
 270useful to have UAPI changes properly designed to be usable, but also
 271to make those changes available to a wider user base of major
 272downstream distributions.
 273
 274Q: Do you accept patches as well for iproute2's BPF loader?
 275-----------------------------------------------------------
 276A: Patches for the iproute2's BPF loader have to be sent to:
 277
 278  netdev@vger.kernel.org
 279
 280While those patches are not processed by the BPF kernel maintainers,
 281please keep them in Cc as well, so they can be reviewed.
 282
 283The official git repository for iproute2 is run by Stephen Hemminger
 284and can be found at:
 285
 286  https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/
 287
 288The patches need to have a subject prefix of '``[PATCH iproute2
 289master]``' or '``[PATCH iproute2 net-next]``'. '``master``' or
 290'``net-next``' describes the target branch where the patch should be
 291applied to. Meaning, if kernel changes went into the net-next kernel
 292tree, then the related iproute2 changes need to go into the iproute2
 293net-next branch, otherwise they can be targeted at master branch. The
 294iproute2 net-next branch will get merged into the master branch after
 295the current iproute2 version from master has been released.
 296
 297Like BPF, the patches end up in patchwork under the netdev project and
 298are delegated to 'shemminger' for further processing:
 299
 300  http://patchwork.ozlabs.org/project/netdev/list/?delegate=389
 301
 302Q: What is the minimum requirement before I submit my BPF patches?
 303------------------------------------------------------------------
 304A: When submitting patches, always take the time and properly test your
 305patches *prior* to submission. Never rush them! If maintainers find
 306that your patches have not been properly tested, it is a good way to
 307get them grumpy. Testing patch submissions is a hard requirement!
 308
 309Note, fixes that go to bpf tree *must* have a ``Fixes:`` tag included.
 310The same applies to fixes that target bpf-next, where the affected
 311commit is in net-next (or in some cases bpf-next). The ``Fixes:`` tag is
 312crucial in order to identify follow-up commits and tremendously helps
 313for people having to do backporting, so it is a must have!
 314
 315We also don't accept patches with an empty commit message. Take your
 316time and properly write up a high quality commit message, it is
 317essential!
 318
 319Think about it this way: other developers looking at your code a month
 320from now need to understand *why* a certain change has been done that
 321way, and whether there have been flaws in the analysis or assumptions
 322that the original author did. Thus providing a proper rationale and
 323describing the use-case for the changes is a must.
 324
 325Patch submissions with >1 patch must have a cover letter which includes
 326a high level description of the series. This high level summary will
 327then be placed into the merge commit by the BPF maintainers such that
 328it is also accessible from the git log for future reference.
 329
 330Q: Features changing BPF JIT and/or LLVM
 331----------------------------------------
 332Q: What do I need to consider when adding a new instruction or feature
 333that would require BPF JIT and/or LLVM integration as well?
 334
 335A: We try hard to keep all BPF JITs up to date such that the same user
 336experience can be guaranteed when running BPF programs on different
 337architectures without having the program punt to the less efficient
 338interpreter in case the in-kernel BPF JIT is enabled.
 339
 340If you are unable to implement or test the required JIT changes for
 341certain architectures, please work together with the related BPF JIT
 342developers in order to get the feature implemented in a timely manner.
 343Please refer to the git log (``arch/*/net/``) to locate the necessary
 344people for helping out.
 345
 346Also always make sure to add BPF test cases (e.g. test_bpf.c and
 347test_verifier.c) for new instructions, so that they can receive
 348broad test coverage and help run-time testing the various BPF JITs.
 349
 350In case of new BPF instructions, once the changes have been accepted
 351into the Linux kernel, please implement support into LLVM's BPF back
 352end. See LLVM_ section below for further information.
 353
 354Stable submission
 355=================
 356
 357Q: I need a specific BPF commit in stable kernels. What should I do?
 358--------------------------------------------------------------------
 359A: In case you need a specific fix in stable kernels, first check whether
 360the commit has already been applied in the related ``linux-*.y`` branches:
 361
 362  https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/
 363
 364If not the case, then drop an email to the BPF maintainers with the
 365netdev kernel mailing list in Cc and ask for the fix to be queued up:
 366
 367  netdev@vger.kernel.org
 368
 369The process in general is the same as on netdev itself, see also the
 370:ref:`netdev-FAQ`.
 371
 372Q: Do you also backport to kernels not currently maintained as stable?
 373----------------------------------------------------------------------
 374A: No. If you need a specific BPF commit in kernels that are currently not
 375maintained by the stable maintainers, then you are on your own.
 376
 377The current stable and longterm stable kernels are all listed here:
 378
 379  https://www.kernel.org/
 380
 381Q: The BPF patch I am about to submit needs to go to stable as well
 382-------------------------------------------------------------------
 383What should I do?
 384
 385A: The same rules apply as with netdev patch submissions in general, see
 386the :ref:`netdev-FAQ`.
 387
 388Never add "``Cc: stable@vger.kernel.org``" to the patch description, but
 389ask the BPF maintainers to queue the patches instead. This can be done
 390with a note, for example, under the ``---`` part of the patch which does
 391not go into the git log. Alternatively, this can be done as a simple
 392request by mail instead.
 393
 394Q: Queue stable patches
 395-----------------------
 396Q: Where do I find currently queued BPF patches that will be submitted
 397to stable?
 398
 399A: Once patches that fix critical bugs got applied into the bpf tree, they
 400are queued up for stable submission under:
 401
 402  http://patchwork.ozlabs.org/bundle/bpf/stable/?state=*
 403
 404They will be on hold there at minimum until the related commit made its
 405way into the mainline kernel tree.
 406
 407After having been under broader exposure, the queued patches will be
 408submitted by the BPF maintainers to the stable maintainers.
 409
 410Testing patches
 411===============
 412
 413Q: How to run BPF selftests
 414---------------------------
 415A: After you have booted into the newly compiled kernel, navigate to
 416the BPF selftests_ suite in order to test BPF functionality (current
 417working directory points to the root of the cloned git tree)::
 418
 419  $ cd tools/testing/selftests/bpf/
 420  $ make
 421
 422To run the verifier tests::
 423
 424  $ sudo ./test_verifier
 425
 426The verifier tests print out all the current checks being
 427performed. The summary at the end of running all tests will dump
 428information of test successes and failures::
 429
 430  Summary: 418 PASSED, 0 FAILED
 431
 432In order to run through all BPF selftests, the following command is
 433needed::
 434
 435  $ sudo make run_tests
 436
 437See the kernels selftest `Documentation/dev-tools/kselftest.rst`_
 438document for further documentation.
 439
 440To maximize the number of tests passing, the .config of the kernel
 441under test should match the config file fragment in
 442tools/testing/selftests/bpf as closely as possible.
 443
 444Finally to ensure support for latest BPF Type Format features -
 445discussed in `Documentation/bpf/btf.rst`_ - pahole version 1.16
 446is required for kernels built with CONFIG_DEBUG_INFO_BTF=y.
 447pahole is delivered in the dwarves package or can be built
 448from source at
 449
 450https://github.com/acmel/dwarves
 451
 452pahole starts to use libbpf definitions and APIs since v1.13 after the
 453commit 21507cd3e97b ("pahole: add libbpf as submodule under lib/bpf").
 454It works well with the git repository because the libbpf submodule will
 455use "git submodule update --init --recursive" to update.
 456
 457Unfortunately, the default github release source code does not contain
 458libbpf submodule source code and this will cause build issues, the tarball
 459from https://git.kernel.org/pub/scm/devel/pahole/pahole.git/ is same with
 460github, you can get the source tarball with corresponding libbpf submodule
 461codes from
 462
 463https://fedorapeople.org/~acme/dwarves
 464
 465Some distros have pahole version 1.16 packaged already, e.g.
 466Fedora, Gentoo.
 467
 468Q: Which BPF kernel selftests version should I run my kernel against?
 469---------------------------------------------------------------------
 470A: If you run a kernel ``xyz``, then always run the BPF kernel selftests
 471from that kernel ``xyz`` as well. Do not expect that the BPF selftest
 472from the latest mainline tree will pass all the time.
 473
 474In particular, test_bpf.c and test_verifier.c have a large number of
 475test cases and are constantly updated with new BPF test sequences, or
 476existing ones are adapted to verifier changes e.g. due to verifier
 477becoming smarter and being able to better track certain things.
 478
 479LLVM
 480====
 481
 482Q: Where do I find LLVM with BPF support?
 483-----------------------------------------
 484A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.
 485
 486All major distributions these days ship LLVM with BPF back end enabled,
 487so for the majority of use-cases it is not required to compile LLVM by
 488hand anymore, just install the distribution provided package.
 489
 490LLVM's static compiler lists the supported targets through
 491``llc --version``, make sure BPF targets are listed. Example::
 492
 493     $ llc --version
 494     LLVM (http://llvm.org/):
 495       LLVM version 10.0.0
 496       Optimized build.
 497       Default target: x86_64-unknown-linux-gnu
 498       Host CPU: skylake
 499
 500       Registered Targets:
 501         aarch64    - AArch64 (little endian)
 502         bpf        - BPF (host endian)
 503         bpfeb      - BPF (big endian)
 504         bpfel      - BPF (little endian)
 505         x86        - 32-bit X86: Pentium-Pro and above
 506         x86-64     - 64-bit X86: EM64T and AMD64
 507
 508For developers in order to utilize the latest features added to LLVM's
 509BPF back end, it is advisable to run the latest LLVM releases. Support
 510for new BPF kernel features such as additions to the BPF instruction
 511set are often developed together.
 512
 513All LLVM releases can be found at: http://releases.llvm.org/
 514
 515Q: Got it, so how do I build LLVM manually anyway?
 516--------------------------------------------------
 517A: We recommend that developers who want the fastest incremental builds
 518use the Ninja build system, you can find it in your system's package
 519manager, usually the package is ninja or ninja-build.
 520
 521You need ninja, cmake and gcc-c++ as build requisites for LLVM. Once you
 522have that set up, proceed with building the latest LLVM and clang version
 523from the git repositories::
 524
 525     $ git clone https://github.com/llvm/llvm-project.git
 526     $ mkdir -p llvm-project/llvm/build
 527     $ cd llvm-project/llvm/build
 528     $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
 529                -DLLVM_ENABLE_PROJECTS="clang"    \
 530                -DCMAKE_BUILD_TYPE=Release        \
 531                -DLLVM_BUILD_RUNTIME=OFF
 532     $ ninja
 533
 534The built binaries can then be found in the build/bin/ directory, where
 535you can point the PATH variable to.
 536
 537Set ``-DLLVM_TARGETS_TO_BUILD`` equal to the target you wish to build, you
 538will find a full list of targets within the llvm-project/llvm/lib/Target
 539directory.
 540
 541Q: Reporting LLVM BPF issues
 542----------------------------
 543Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code
 544generation back end or about LLVM generated code that the verifier
 545refuses to accept?
 546
 547A: Yes, please do!
 548
 549LLVM's BPF back end is a key piece of the whole BPF
 550infrastructure and it ties deeply into verification of programs from the
 551kernel side. Therefore, any issues on either side need to be investigated
 552and fixed whenever necessary.
 553
 554Therefore, please make sure to bring them up at netdev kernel mailing
 555list and Cc BPF maintainers for LLVM and kernel bits:
 556
 557* Yonghong Song <yhs@fb.com>
 558* Alexei Starovoitov <ast@kernel.org>
 559* Daniel Borkmann <daniel@iogearbox.net>
 560
 561LLVM also has an issue tracker where BPF related bugs can be found:
 562
 563  https://bugs.llvm.org/buglist.cgi?quicksearch=bpf
 564
 565However, it is better to reach out through mailing lists with having
 566maintainers in Cc.
 567
 568Q: New BPF instruction for kernel and LLVM
 569------------------------------------------
 570Q: I have added a new BPF instruction to the kernel, how can I integrate
 571it into LLVM?
 572
 573A: LLVM has a ``-mcpu`` selector for the BPF back end in order to allow
 574the selection of BPF instruction set extensions. By default the
 575``generic`` processor target is used, which is the base instruction set
 576(v1) of BPF.
 577
 578LLVM has an option to select ``-mcpu=probe`` where it will probe the host
 579kernel for supported BPF instruction set extensions and selects the
 580optimal set automatically.
 581
 582For cross-compilation, a specific version can be select manually as well ::
 583
 584     $ llc -march bpf -mcpu=help
 585     Available CPUs for this target:
 586
 587       generic - Select the generic processor.
 588       probe   - Select the probe processor.
 589       v1      - Select the v1 processor.
 590       v2      - Select the v2 processor.
 591     [...]
 592
 593Newly added BPF instructions to the Linux kernel need to follow the same
 594scheme, bump the instruction set version and implement probing for the
 595extensions such that ``-mcpu=probe`` users can benefit from the
 596optimization transparently when upgrading their kernels.
 597
 598If you are unable to implement support for the newly added BPF instruction
 599please reach out to BPF developers for help.
 600
 601By the way, the BPF kernel selftests run with ``-mcpu=probe`` for better
 602test coverage.
 603
 604Q: clang flag for target bpf?
 605-----------------------------
 606Q: In some cases clang flag ``-target bpf`` is used but in other cases the
 607default clang target, which matches the underlying architecture, is used.
 608What is the difference and when I should use which?
 609
 610A: Although LLVM IR generation and optimization try to stay architecture
 611independent, ``-target <arch>`` still has some impact on generated code:
 612
 613- BPF program may recursively include header file(s) with file scope
 614  inline assembly codes. The default target can handle this well,
 615  while ``bpf`` target may fail if bpf backend assembler does not
 616  understand these assembly codes, which is true in most cases.
 617
 618- When compiled without ``-g``, additional elf sections, e.g.,
 619  .eh_frame and .rela.eh_frame, may be present in the object file
 620  with default target, but not with ``bpf`` target.
 621
 622- The default target may turn a C switch statement into a switch table
 623  lookup and jump operation. Since the switch table is placed
 624  in the global readonly section, the bpf program will fail to load.
 625  The bpf target does not support switch table optimization.
 626  The clang option ``-fno-jump-tables`` can be used to disable
 627  switch table generation.
 628
 629- For clang ``-target bpf``, it is guaranteed that pointer or long /
 630  unsigned long types will always have a width of 64 bit, no matter
 631  whether underlying clang binary or default target (or kernel) is
 632  32 bit. However, when native clang target is used, then it will
 633  compile these types based on the underlying architecture's conventions,
 634  meaning in case of 32 bit architecture, pointer or long / unsigned
 635  long types e.g. in BPF context structure will have width of 32 bit
 636  while the BPF LLVM back end still operates in 64 bit. The native
 637  target is mostly needed in tracing for the case of walking ``pt_regs``
 638  or other kernel structures where CPU's register width matters.
 639  Otherwise, ``clang -target bpf`` is generally recommended.
 640
 641You should use default target when:
 642
 643- Your program includes a header file, e.g., ptrace.h, which eventually
 644  pulls in some header files containing file scope host assembly codes.
 645
 646- You can add ``-fno-jump-tables`` to work around the switch table issue.
 647
 648Otherwise, you can use ``bpf`` target. Additionally, you *must* use bpf target
 649when:
 650
 651- Your program uses data structures with pointer or long / unsigned long
 652  types that interface with BPF helpers or context data structures. Access
 653  into these structures is verified by the BPF verifier and may result
 654  in verification failures if the native architecture is not aligned with
 655  the BPF architecture, e.g. 64-bit. An example of this is
 656  BPF_PROG_TYPE_SK_MSG require ``-target bpf``
 657
 658
 659.. Links
 660.. _Documentation/process/: https://www.kernel.org/doc/html/latest/process/
 661.. _netdev-FAQ: ../networking/netdev-FAQ.rst
 662.. _selftests:
 663   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/
 664.. _Documentation/dev-tools/kselftest.rst:
 665   https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html
 666.. _Documentation/bpf/btf.rst: btf.rst
 667
 668Happy BPF hacking!
 669