bpf, doc: convert bpf_devel_QA.rst to use RST formatting
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>hifive-unleashed-5.1
Jesper Dangaard Brouer
Alexei Starovoitov
parent
1a6ac1d59d
commit
5422283848
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@ -1,16 +1,24 @@
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=================================
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HOWTO interact with BPF subsystem
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=================================
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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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`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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.. contents::
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:local:
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:depth: 2
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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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--------------------------------------------
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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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@ -21,23 +29,23 @@ A: Since all BPF kernel development as well as bpftool and iproute2 BPF
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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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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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* 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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**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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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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------------------------------------------------------------
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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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@ -53,7 +61,7 @@ A: Please submit your BPF patches to the netdev kernel mailing list:
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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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-------------------------------------------------------------------------
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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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@ -78,13 +86,13 @@ A: All patches that are Cc'ed to netdev are queued for review under netdev
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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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------------------------------------------------
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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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* 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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@ -98,9 +106,9 @@ A: There are two BPF kernel trees (git repositories). Once patches have
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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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the `netdev FAQ`_ under:
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Documentation/networking/netdev-FAQ.txt
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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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@ -108,26 +116,27 @@ A: There are two BPF kernel trees (git repositories). Once patches have
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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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following subject lines (``yyyy-mm-dd`` is the date of the pull
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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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Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be applied to?
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---------------------------------------------------------------------------------
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A: The process is the very same as described in the netdev FAQ, so
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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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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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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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@ -143,7 +152,7 @@ A: The process is the very same as described in the netdev FAQ, so
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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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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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@ -152,7 +161,7 @@ A: The process is the very same as described in the netdev FAQ, so
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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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-----------------------------------------------------------------------
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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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@ -164,24 +173,28 @@ A: It means that the patch looks good for mainline inclusion from
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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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is to:
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i) accumulate and stage BPF patches for integration into trees
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like net and net-next, and
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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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`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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-------------------------------------------------------------
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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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Q: How often do you send pull requests to major kernel trees like net or net-next?
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----------------------------------------------------------------------------------
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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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@ -192,26 +205,28 @@ A: Pull requests will be sent out rather often in order to not
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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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-----------------------------------------------------------------
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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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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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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: Verifier changes and test cases
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----------------------------------
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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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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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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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@ -222,11 +237,13 @@ A: If the patch has changes to the behavior of the verifier, then yes,
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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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Q: samples/bpf preference vs selftests?
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---------------------------------------
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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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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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@ -234,16 +251,16 @@ A: In general, we prefer additions to BPF kernel selftests rather than
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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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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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`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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-----------------------------------------
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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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@ -251,16 +268,16 @@ A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
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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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---------------------------------------------------
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A: For UAPI changes related to the XDP or tc layer (e.g. ``cls_bpf``),
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the 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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-----------------------------------------------------------
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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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@ -273,14 +290,14 @@ A: Patches for the iproute2's BPF loader have to be sent to:
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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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The patches need to have a subject prefix of '``[PATCH iproute2
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master]``' or '``[PATCH iproute2 net-next]``'. '``master``' or
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'``net-next``' describes the target branch where the patch should be
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applied to. Meaning, if kernel changes went into the net-next kernel
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tree, then the related iproute2 changes need to go into the iproute2
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net-next branch, otherwise they can be targeted at master branch. The
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iproute2 net-next branch will get merged into the master branch after
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the current iproute2 version from 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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@ -288,17 +305,17 @@ A: Patches for the iproute2's BPF loader have to be sent to:
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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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------------------------------------------------------------------
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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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Note, fixes that go to bpf tree *must* have a ``Fixes:`` tag included.
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The same applies to fixes that target bpf-next, where the affected
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commit is in net-next (or in some cases bpf-next). The ``Fixes:`` tag is
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crucial in order to identify follow-up commits and tremendously helps
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for people 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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@ -315,6 +332,8 @@ A: When submitting patches, always take the time and properly test your
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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: Features changing BPF JIT and/or LLVM
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----------------------------------------
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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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@ -326,7 +345,7 @@ A: We try hard to keep all BPF JITs up to date such that the same user
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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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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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@ -335,15 +354,15 @@ A: We try hard to keep all BPF JITs up to date such that the same user
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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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end. See LLVM_ section below for further information.
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Stable submission:
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------------------
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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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--------------------------------------------------------------------
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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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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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@ -353,10 +372,10 @@ A: In case you need a specific fix in stable kernels, first check whether
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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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`netdev FAQ`_ document.
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Q: Do you also backport to kernels not currently maintained as stable?
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----------------------------------------------------------------------
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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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@ -364,20 +383,23 @@ A: No. If you need a specific BPF commit in kernels that are currently not
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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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Q: The BPF patch I am about to submit needs to go to stable as well
|
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-------------------------------------------------------------------
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What 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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`netdev FAQ`_ under:
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Documentation/networking/netdev-FAQ.txt
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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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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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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: Queue stable patches
|
||||
-----------------------
|
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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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|
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|
|
@ -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!
|
||||
|
|
|
|||
Loading…
Reference in New Issue