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