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bpf, doc: convert bpf_devel_QA.rst to use RST formatting

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Same story as bpf_design_QA.rst RST format conversion.

Again thanks to Quentin Monnet <quentin.monnet@netronome.com> for
fixes and patches that have been squashed.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Jesper Dangaard Brouer 2018-05-14 15:42:27 +02:00 committed by Alexei Starovoitov
parent 1a6ac1d59d
commit 5422283848
1 changed files with 398 additions and 357 deletions
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267
Documentation/bpf/bpf_devel_QA.rst
View file

@ -1,16 +1,24 @@
=================================
HOWTO interact with BPF subsystem
=================================
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
`Documentation/process/`_. This document only describes additional specifics
related to BPF.
Reporting bugs:
---------------
.. contents::
:local:
:depth: 2
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
@ -21,23 +29,23 @@ A: Since all BPF kernel development as well as bpftool and iproute2 BPF
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):
maintainers to Cc (from kernel MAINTAINERS_ file):
Alexei Starovoitov <ast@kernel.org>
Daniel Borkmann <daniel@iogearbox.net>
* 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.
**Please do NOT report BPF issues to bugzilla.kernel.org since it
is a guarantee that the reported issue will be overlooked.**
Submitting patches:
-------------------
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
@ -53,7 +61,7 @@ A: Please submit your BPF patches to the netdev kernel mailing list:
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:
@ -78,13 +86,13 @@ A: All patches that are Cc'ed to netdev are queued for review under netdev
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/
* 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
@ -98,9 +106,9 @@ A: There are two BPF kernel trees (git repositories). Once patches have
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:
the `netdev FAQ`_ under:
Documentation/networking/netdev-FAQ.txt
`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
@ -108,26 +116,27 @@ A: There are two BPF kernel trees (git repositories). Once patches have
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):
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?
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
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:
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:
bpf-next -> net-next, the subject must look like::
git format-patch --subject-prefix='PATCH bpf-next' start..finish
@ -143,7 +152,7 @@ A: The process is the very same as described in the netdev FAQ, so
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:
version number (``v2``, ``v3``, ...) into the subject prefix::
git format-patch --subject-prefix='PATCH net-next v2' start..finish
@ -152,7 +161,7 @@ A: The process is the very same as described in the netdev FAQ, so
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.
@ -164,24 +173,28 @@ A: It means that the patch looks good for mainline inclusion from
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
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
`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?
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.
@ -192,26 +205,28 @@ A: Pull requests will be sent out rather often in order to not
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.
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.
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: Verifier changes and test cases
----------------------------------
Q: I made a BPF verifier change, do I need to add test cases for
BPF kernel selftests?
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
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
@ -222,11 +237,13 @@ A: If the patch has changes to the behavior of the verifier, then yes,
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?
Q: samples/bpf preference vs selftests?
---------------------------------------
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
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
@ -234,16 +251,16 @@ A: In general, we prefer additions to BPF kernel selftests rather than
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,
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
`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
@ -251,16 +268,16 @@ A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
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
---------------------------------------------------
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
@ -273,14 +290,14 @@ A: Patches for the iproute2's BPF loader have to be sent to:
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.
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:
@ -288,17 +305,17 @@ A: Patches for the iproute2's BPF loader have to be sent to:
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!
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
@ -315,6 +332,8 @@ A: When submitting patches, always take the time and properly test your
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: Features changing BPF JIT and/or LLVM
----------------------------------------
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?
@ -326,7 +345,7 @@ A: We try hard to keep all BPF JITs up to date such that the same user
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
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
@ -335,15 +354,15 @@ A: We try hard to keep all BPF JITs up to date such that the same user
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.
end. See LLVM_ section below for further information.
Stable submission:
------------------
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:
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/
@ -353,10 +372,10 @@ A: In case you need a specific fix in stable kernels, first check whether
netdev@vger.kernel.org
The process in general is the same as on netdev itself, see also the
netdev FAQ document.
`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.
@ -364,20 +383,23 @@ A: No. If you need a specific BPF commit in kernels that are currently not
https://www.kernel.org/
Q: The BPF patch I am about to submit needs to go to stable as well. What
should I do?
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:
`netdev FAQ`_ under:
Documentation/networking/netdev-FAQ.txt
`Documentation/networking/netdev-FAQ.txt`_
Never add "Cc: stable@vger.kernel.org" to the patch description, but
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
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: Queue stable patches
-----------------------
Q: Where do I find currently queued BPF patches that will be submitted
to stable?
@ -392,33 +414,33 @@ A: Once patches that fix critical bugs got applied into the bpf tree, they
After having been under broader exposure, the queued patches will be
submitted by the BPF maintainers to the stable maintainers.
Testing patches:
----------------
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.
---------------------------------------------------------------------
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:
-----
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:
LLVM's static compiler lists the supported targets through
``llc --version``, make sure BPF targets are listed. Example::
$ llc --version
LLVM (http://llvm.org/):
@ -442,10 +464,10 @@ A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.
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:
from the git repositories::
$ git clone http://llvm.org/git/llvm.git
$ cd llvm/tools
@ -460,11 +482,15 @@ A: You need cmake and gcc-c++ as build requisites for LLVM. Once you have
The built binaries can then be found in the build/bin/ directory, where
you can point the PATH variable to.
Q: Reporting LLVM BPF issues
----------------------------
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
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.
@ -472,9 +498,9 @@ A: Yes, please do! LLVM's BPF back end is a key piece of the whole BPF
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>
* 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:
@ -483,18 +509,21 @@ A: Yes, please do! LLVM's BPF back end is a key piece of the whole BPF
However, it is better to reach out through mailing lists with having
maintainers in Cc.
Q: New BPF instruction for kernel and LLVM
------------------------------------------
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.
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
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.
For cross-compilation, a specific version can be select manually as well ::
$ llc -march bpf -mcpu=help
Available CPUs for this target:
@ -507,39 +536,41 @@ A: LLVM has a -mcpu selector for the BPF back end in order to allow the
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.
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
By the way, the BPF kernel selftests run with ``-mcpu=probe`` for better
test coverage.
Q: In some cases clang flag "-target bpf" is used but in other cases the
Q: clang flag for target bpf?
-----------------------------
Q: In some cases clang flag ``-target bpf`` is used but in other cases the
default clang target, which matches the underlying architecture, is used.
What is the difference and when I should use which?
A: Although LLVM IR generation and optimization try to stay architecture
independent, "-target <arch>" still has some impact on generated code:
independent, ``-target <arch>`` still has some impact on generated code:
- BPF program may recursively include header file(s) with file scope
inline assembly codes. The default target can handle this well,
while bpf target may fail if bpf backend assembler does not
while ``bpf`` target may fail if bpf backend assembler does not
understand these assembly codes, which is true in most cases.
- When compiled without -g, additional elf sections, e.g.,
- When compiled without ``-g``, additional elf sections, e.g.,
.eh_frame and .rela.eh_frame, may be present in the object file
with default target, but not with bpf target.
with default target, but not with ``bpf`` target.
- The default target may turn a C switch statement into a switch table
lookup and jump operation. Since the switch table is placed
in the global readonly section, the bpf program will fail to load.
The bpf target does not support switch table optimization.
The clang option "-fno-jump-tables" can be used to disable
The clang option ``-fno-jump-tables`` can be used to disable
switch table generation.
- For clang -target bpf, it is guaranteed that pointer or long /
- For clang ``-target bpf``, it is guaranteed that pointer or long /
unsigned long types will always have a width of 64 bit, no matter
whether underlying clang binary or default target (or kernel) is
32 bit. However, when native clang target is used, then it will
@ -547,17 +578,18 @@ A: Although LLVM IR generation and optimization try to stay architecture
meaning in case of 32 bit architecture, pointer or long / unsigned
long types e.g. in BPF context structure will have width of 32 bit
while the BPF LLVM back end still operates in 64 bit. The native
target is mostly needed in tracing for the case of walking pt_regs
target is mostly needed in tracing for the case of walking ``pt_regs``
or other kernel structures where CPU's register width matters.
Otherwise, clang -target bpf is generally recommended.
Otherwise, ``clang -target bpf`` is generally recommended.
You should use default target when:
- Your program includes a header file, e.g., ptrace.h, which eventually
pulls in some header files containing file scope host assembly codes.
- You can add "-fno-jump-tables" to work around the switch table issue.
Otherwise, you can use bpf target. Additionally, you _must_ use bpf target
- You can add ``-fno-jump-tables`` to work around the switch table issue.
Otherwise, you can use ``bpf`` target. Additionally, you *must* use bpf target
when:
- Your program uses data structures with pointer or long / unsigned long
@ -565,6 +597,15 @@ A: Although LLVM IR generation and optimization try to stay architecture
into these structures is verified by the BPF verifier and may result
in verification failures if the native architecture is not aligned with
the BPF architecture, e.g. 64-bit. An example of this is
BPF_PROG_TYPE_SK_MSG require '-target bpf'
BPF_PROG_TYPE_SK_MSG require ``-target bpf``
.. Links
.. _Documentation/process/: https://www.kernel.org/doc/html/latest/process/
.. _MAINTAINERS: ../../MAINTAINERS
.. _Documentation/networking/netdev-FAQ.txt: ../networking/netdev-FAQ.txt
.. _netdev FAQ: ../networking/netdev-FAQ.txt
.. _samples/bpf/: ../../samples/bpf/
.. _selftests: ../../tools/testing/selftests/bpf/
Happy BPF hacking!