This will cause them to get auto rendered, e.g. when viewing them on GitHub. Followup patches will correct the content to be RST compliant. Also adjust README.rst to point to the renamed files. Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx> --- Documentation/bpf/README.rst | 4 Documentation/bpf/bpf_design_QA.rst | 156 ++++++++++ Documentation/bpf/bpf_design_QA.txt | 156 ---------- Documentation/bpf/bpf_devel_QA.rst | 570 +++++++++++++++++++++++++++++++++++ Documentation/bpf/bpf_devel_QA.txt | 570 ----------------------------------- 5 files changed, 728 insertions(+), 728 deletions(-) create mode 100644 Documentation/bpf/bpf_design_QA.rst delete mode 100644 Documentation/bpf/bpf_design_QA.txt create mode 100644 Documentation/bpf/bpf_devel_QA.rst delete mode 100644 Documentation/bpf/bpf_devel_QA.txt diff --git a/Documentation/bpf/README.rst b/Documentation/bpf/README.rst index 329469c33db8..b9a80c9e9392 100644 --- a/Documentation/bpf/README.rst +++ b/Documentation/bpf/README.rst @@ -28,8 +28,8 @@ Two sets of Questions and Answers (Q&A) are maintained. .. Links: -.. _bpf_design_QA: bpf_design_QA.txt -.. _bpf_devel_QA: bpf_devel_QA.txt +.. _bpf_design_QA: bpf_design_QA.rst +.. _bpf_devel_QA: bpf_devel_QA.rst .. _Documentation/networking/filter.txt: ../networking/filter.txt .. _man-pages: https://www.kernel.org/doc/man-pages/ .. _bpf(2): http://man7.org/linux/man-pages/man2/bpf.2.html diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst new file mode 100644 index 000000000000..f3e458a0bb2f --- /dev/null +++ b/Documentation/bpf/bpf_design_QA.rst @@ -0,0 +1,156 @@ +BPF extensibility and applicability to networking, tracing, security +in the linux kernel and several user space implementations of BPF +virtual machine led to a number of misunderstanding on what BPF actually is. +This short QA is an attempt to address that and outline a direction +of where BPF is heading long term. + +Q: Is BPF a generic instruction set similar to x64 and arm64? +A: NO. + +Q: Is BPF a generic virtual machine ? +A: NO. + +BPF is generic instruction set _with_ C calling convention. + +Q: Why C calling convention was chosen? +A: Because BPF programs are designed to run in the linux kernel + which is written in C, hence BPF defines instruction set compatible + with two most used architectures x64 and arm64 (and takes into + consideration important quirks of other architectures) and + defines calling convention that is compatible with C calling + convention of the linux kernel on those architectures. + +Q: can multiple return values be supported in the future? +A: NO. BPF allows only register R0 to be used as return value. + +Q: can more than 5 function arguments be supported in the future? +A: NO. BPF calling convention only allows registers R1-R5 to be used + as arguments. BPF is not a standalone instruction set. + (unlike x64 ISA that allows msft, cdecl and other conventions) + +Q: can BPF programs access instruction pointer or return address? +A: NO. + +Q: can BPF programs access stack pointer ? +A: NO. Only frame pointer (register R10) is accessible. + From compiler point of view it's necessary to have stack pointer. + For example LLVM defines register R11 as stack pointer in its + BPF backend, but it makes sure that generated code never uses it. + +Q: Does C-calling convention diminishes possible use cases? +A: YES. BPF design forces addition of major functionality in the form + of kernel helper functions and kernel objects like BPF maps with + seamless interoperability between them. It lets kernel call into + BPF programs and programs call kernel helpers with zero overhead. + As all of them were native C code. That is particularly the case + for JITed BPF programs that are indistinguishable from + native kernel C code. + +Q: Does it mean that 'innovative' extensions to BPF code are disallowed? +A: Soft yes. At least for now until BPF core has support for + bpf-to-bpf calls, indirect calls, loops, global variables, + jump tables, read only sections and all other normal constructs + that C code can produce. + +Q: Can loops be supported in a safe way? +A: It's not clear yet. BPF developers are trying to find a way to + support bounded loops where the verifier can guarantee that + the program terminates in less than 4096 instructions. + +Q: How come LD_ABS and LD_IND instruction are present in BPF whereas + C code cannot express them and has to use builtin intrinsics? +A: This is artifact of compatibility with classic BPF. Modern + networking code in BPF performs better without them. + See 'direct packet access'. + +Q: It seems not all BPF instructions are one-to-one to native CPU. + For example why BPF_JNE and other compare and jumps are not cpu-like? +A: This was necessary to avoid introducing flags into ISA which are + impossible to make generic and efficient across CPU architectures. + +Q: why BPF_DIV instruction doesn't map to x64 div? +A: Because if we picked one-to-one relationship to x64 it would have made + it more complicated to support on arm64 and other archs. Also it + needs div-by-zero runtime check. + +Q: why there is no BPF_SDIV for signed divide operation? +A: Because it would be rarely used. llvm errors in such case and + prints a suggestion to use unsigned divide instead + +Q: Why BPF has implicit prologue and epilogue? +A: Because architectures like sparc have register windows and in general + there are enough subtle differences between architectures, so naive + store return address into stack won't work. Another reason is BPF has + to be safe from division by zero (and legacy exception path + of LD_ABS insn). Those instructions need to invoke epilogue and + return implicitly. + +Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning? +A: Because classic BPF didn't have them and BPF authors felt that compiler + workaround would be acceptable. Turned out that programs lose performance + due to lack of these compare instructions and they were added. + These two instructions is a perfect example what kind of new BPF + instructions are acceptable and can be added in the future. + These two already had equivalent instructions in native CPUs. + New instructions that don't have one-to-one mapping to HW instructions + will not be accepted. + +Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF + registers which makes BPF inefficient virtual machine for 32-bit + CPU architectures and 32-bit HW accelerators. Can true 32-bit registers + be added to BPF in the future? +A: NO. The first thing to improve performance on 32-bit archs is to teach + LLVM to generate code that uses 32-bit subregisters. Then second step + is to teach verifier to mark operations where zero-ing upper bits + is unnecessary. Then JITs can take advantage of those markings and + drastically reduce size of generated code and improve performance. + +Q: Does BPF have a stable ABI? +A: YES. BPF instructions, arguments to BPF programs, set of helper + functions and their arguments, recognized return codes are all part + of ABI. However when tracing programs are using bpf_probe_read() helper + to walk kernel internal datastructures and compile with kernel + internal headers these accesses can and will break with newer + kernels. The union bpf_attr -> kern_version is checked at load time + to prevent accidentally loading kprobe-based bpf programs written + for a different kernel. Networking programs don't do kern_version check. + +Q: How much stack space a BPF program uses? +A: Currently all program types are limited to 512 bytes of stack + space, but the verifier computes the actual amount of stack used + and both interpreter and most JITed code consume necessary amount. + +Q: Can BPF be offloaded to HW? +A: YES. BPF HW offload is supported by NFP driver. + +Q: Does classic BPF interpreter still exist? +A: NO. Classic BPF programs are converted into extend BPF instructions. + +Q: Can BPF call arbitrary kernel functions? +A: NO. BPF programs can only call a set of helper functions which + is defined for every program type. + +Q: Can BPF overwrite arbitrary kernel memory? +A: NO. Tracing bpf programs can _read_ arbitrary memory with bpf_probe_read() + and bpf_probe_read_str() helpers. Networking programs cannot read + arbitrary memory, since they don't have access to these helpers. + Programs can never read or write arbitrary memory directly. + +Q: Can BPF overwrite arbitrary user memory? +A: Sort-of. Tracing BPF programs can overwrite the user memory + of the current task with bpf_probe_write_user(). Every time such + program is loaded the kernel will print warning message, so + this helper is only useful for experiments and prototypes. + Tracing BPF programs are root only. + +Q: When bpf_trace_printk() helper is used the kernel prints nasty + warning message. Why is that? +A: This is done to nudge program authors into better interfaces when + programs need to pass data to user space. Like bpf_perf_event_output() + can be used to efficiently stream data via perf ring buffer. + BPF maps can be used for asynchronous data sharing between kernel + and user space. bpf_trace_printk() should only be used for debugging. + +Q: Can BPF functionality such as new program or map types, new + helpers, etc be added out of kernel module code? +A: NO. diff --git a/Documentation/bpf/bpf_design_QA.txt b/Documentation/bpf/bpf_design_QA.txt deleted file mode 100644 index f3e458a0bb2f..000000000000 --- a/Documentation/bpf/bpf_design_QA.txt +++ /dev/null @@ -1,156 +0,0 @@ -BPF extensibility and applicability to networking, tracing, security -in the linux kernel and several user space implementations of BPF -virtual machine led to a number of misunderstanding on what BPF actually is. -This short QA is an attempt to address that and outline a direction -of where BPF is heading long term. - -Q: Is BPF a generic instruction set similar to x64 and arm64? -A: NO. - -Q: Is BPF a generic virtual machine ? -A: NO. - -BPF is generic instruction set _with_ C calling convention. - -Q: Why C calling convention was chosen? -A: Because BPF programs are designed to run in the linux kernel - which is written in C, hence BPF defines instruction set compatible - with two most used architectures x64 and arm64 (and takes into - consideration important quirks of other architectures) and - defines calling convention that is compatible with C calling - convention of the linux kernel on those architectures. - -Q: can multiple return values be supported in the future? -A: NO. BPF allows only register R0 to be used as return value. - -Q: can more than 5 function arguments be supported in the future? -A: NO. BPF calling convention only allows registers R1-R5 to be used - as arguments. BPF is not a standalone instruction set. - (unlike x64 ISA that allows msft, cdecl and other conventions) - -Q: can BPF programs access instruction pointer or return address? -A: NO. - -Q: can BPF programs access stack pointer ? -A: NO. Only frame pointer (register R10) is accessible. - From compiler point of view it's necessary to have stack pointer. - For example LLVM defines register R11 as stack pointer in its - BPF backend, but it makes sure that generated code never uses it. - -Q: Does C-calling convention diminishes possible use cases? -A: YES. BPF design forces addition of major functionality in the form - of kernel helper functions and kernel objects like BPF maps with - seamless interoperability between them. It lets kernel call into - BPF programs and programs call kernel helpers with zero overhead. - As all of them were native C code. That is particularly the case - for JITed BPF programs that are indistinguishable from - native kernel C code. - -Q: Does it mean that 'innovative' extensions to BPF code are disallowed? -A: Soft yes. At least for now until BPF core has support for - bpf-to-bpf calls, indirect calls, loops, global variables, - jump tables, read only sections and all other normal constructs - that C code can produce. - -Q: Can loops be supported in a safe way? -A: It's not clear yet. BPF developers are trying to find a way to - support bounded loops where the verifier can guarantee that - the program terminates in less than 4096 instructions. - -Q: How come LD_ABS and LD_IND instruction are present in BPF whereas - C code cannot express them and has to use builtin intrinsics? -A: This is artifact of compatibility with classic BPF. Modern - networking code in BPF performs better without them. - See 'direct packet access'. - -Q: It seems not all BPF instructions are one-to-one to native CPU. - For example why BPF_JNE and other compare and jumps are not cpu-like? -A: This was necessary to avoid introducing flags into ISA which are - impossible to make generic and efficient across CPU architectures. - -Q: why BPF_DIV instruction doesn't map to x64 div? -A: Because if we picked one-to-one relationship to x64 it would have made - it more complicated to support on arm64 and other archs. Also it - needs div-by-zero runtime check. - -Q: why there is no BPF_SDIV for signed divide operation? -A: Because it would be rarely used. llvm errors in such case and - prints a suggestion to use unsigned divide instead - -Q: Why BPF has implicit prologue and epilogue? -A: Because architectures like sparc have register windows and in general - there are enough subtle differences between architectures, so naive - store return address into stack won't work. Another reason is BPF has - to be safe from division by zero (and legacy exception path - of LD_ABS insn). Those instructions need to invoke epilogue and - return implicitly. - -Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning? -A: Because classic BPF didn't have them and BPF authors felt that compiler - workaround would be acceptable. Turned out that programs lose performance - due to lack of these compare instructions and they were added. - These two instructions is a perfect example what kind of new BPF - instructions are acceptable and can be added in the future. - These two already had equivalent instructions in native CPUs. - New instructions that don't have one-to-one mapping to HW instructions - will not be accepted. - -Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF - registers which makes BPF inefficient virtual machine for 32-bit - CPU architectures and 32-bit HW accelerators. Can true 32-bit registers - be added to BPF in the future? -A: NO. The first thing to improve performance on 32-bit archs is to teach - LLVM to generate code that uses 32-bit subregisters. Then second step - is to teach verifier to mark operations where zero-ing upper bits - is unnecessary. Then JITs can take advantage of those markings and - drastically reduce size of generated code and improve performance. - -Q: Does BPF have a stable ABI? -A: YES. BPF instructions, arguments to BPF programs, set of helper - functions and their arguments, recognized return codes are all part - of ABI. However when tracing programs are using bpf_probe_read() helper - to walk kernel internal datastructures and compile with kernel - internal headers these accesses can and will break with newer - kernels. The union bpf_attr -> kern_version is checked at load time - to prevent accidentally loading kprobe-based bpf programs written - for a different kernel. Networking programs don't do kern_version check. - -Q: How much stack space a BPF program uses? -A: Currently all program types are limited to 512 bytes of stack - space, but the verifier computes the actual amount of stack used - and both interpreter and most JITed code consume necessary amount. - -Q: Can BPF be offloaded to HW? -A: YES. BPF HW offload is supported by NFP driver. - -Q: Does classic BPF interpreter still exist? -A: NO. Classic BPF programs are converted into extend BPF instructions. - -Q: Can BPF call arbitrary kernel functions? -A: NO. BPF programs can only call a set of helper functions which - is defined for every program type. - -Q: Can BPF overwrite arbitrary kernel memory? -A: NO. Tracing bpf programs can _read_ arbitrary memory with bpf_probe_read() - and bpf_probe_read_str() helpers. Networking programs cannot read - arbitrary memory, since they don't have access to these helpers. - Programs can never read or write arbitrary memory directly. - -Q: Can BPF overwrite arbitrary user memory? -A: Sort-of. Tracing BPF programs can overwrite the user memory - of the current task with bpf_probe_write_user(). Every time such - program is loaded the kernel will print warning message, so - this helper is only useful for experiments and prototypes. - Tracing BPF programs are root only. - -Q: When bpf_trace_printk() helper is used the kernel prints nasty - warning message. Why is that? -A: This is done to nudge program authors into better interfaces when - programs need to pass data to user space. Like bpf_perf_event_output() - can be used to efficiently stream data via perf ring buffer. - BPF maps can be used for asynchronous data sharing between kernel - and user space. bpf_trace_printk() should only be used for debugging. - -Q: Can BPF functionality such as new program or map types, new - helpers, etc be added out of kernel module code? -A: NO. diff --git a/Documentation/bpf/bpf_devel_QA.rst b/Documentation/bpf/bpf_devel_QA.rst new file mode 100644 index 000000000000..da57601153a0 --- /dev/null +++ b/Documentation/bpf/bpf_devel_QA.rst @@ -0,0 +1,570 @@ +This document provides information for the BPF subsystem about various +workflows related to reporting bugs, submitting patches, and queueing +patches for stable kernels. + +For general information about submitting patches, please refer to +Documentation/process/. This document only describes additional specifics +related to BPF. + +Reporting bugs: +--------------- + +Q: How do I report bugs for BPF kernel code? + +A: Since all BPF kernel development as well as bpftool and iproute2 BPF + loader development happens through the netdev kernel mailing list, + please report any found issues around BPF to the following mailing + list: + + netdev@xxxxxxxxxxxxxxx + + This may also include issues related to XDP, BPF tracing, etc. + + Given netdev has a high volume of traffic, please also add the BPF + maintainers to Cc (from kernel MAINTAINERS file): + + Alexei Starovoitov <ast@xxxxxxxxxx> + Daniel Borkmann <daniel@xxxxxxxxxxxxx> + + In case a buggy commit has already been identified, make sure to keep + the actual commit authors in Cc as well for the report. They can + typically be identified through the kernel's git tree. + + Please do *not* report BPF issues to bugzilla.kernel.org since it + is a guarantee that the reported issue will be overlooked. + +Submitting patches: +------------------- + +Q: To which mailing list do I need to submit my BPF patches? + +A: Please submit your BPF patches to the netdev kernel mailing list: + + netdev@xxxxxxxxxxxxxxx + + Historically, BPF came out of networking and has always been maintained + by the kernel networking community. Although these days BPF touches + many other subsystems as well, the patches are still routed mainly + through the networking community. + + In case your patch has changes in various different subsystems (e.g. + tracing, security, etc), make sure to Cc the related kernel mailing + lists and maintainers from there as well, so they are able to review + the changes and provide their Acked-by's to the patches. + +Q: Where can I find patches currently under discussion for BPF subsystem? + +A: All patches that are Cc'ed to netdev are queued for review under netdev + patchwork project: + + http://patchwork.ozlabs.org/project/netdev/list/ + + Those patches which target BPF, are assigned to a 'bpf' delegate for + further processing from BPF maintainers. The current queue with + patches under review can be found at: + + https://patchwork.ozlabs.org/project/netdev/list/?delegate=77147 + + Once the patches have been reviewed by the BPF community as a whole + and approved by the BPF maintainers, their status in patchwork will be + changed to 'Accepted' and the submitter will be notified by mail. This + means that the patches look good from a BPF perspective and have been + applied to one of the two BPF kernel trees. + + In case feedback from the community requires a respin of the patches, + their status in patchwork will be set to 'Changes Requested', and purged + from the current review queue. Likewise for cases where patches would + get rejected or are not applicable to the BPF trees (but assigned to + the 'bpf' delegate). + +Q: How do the changes make their way into Linux? + +A: There are two BPF kernel trees (git repositories). Once patches have + been accepted by the BPF maintainers, they will be applied to one + of the two BPF trees: + + https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/ + https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/ + + The bpf tree itself is for fixes only, whereas bpf-next for features, + cleanups or other kind of improvements ("next-like" content). This is + analogous to net and net-next trees for networking. Both bpf and + bpf-next will only have a master branch in order to simplify against + which branch patches should get rebased to. + + Accumulated BPF patches in the bpf tree will regularly get pulled + into the net kernel tree. Likewise, accumulated BPF patches accepted + into the bpf-next tree will make their way into net-next tree. net and + net-next are both run by David S. Miller. From there, they will go + into the kernel mainline tree run by Linus Torvalds. To read up on the + process of net and net-next being merged into the mainline tree, see + the netdev FAQ under: + + Documentation/networking/netdev-FAQ.txt + + Occasionally, to prevent merge conflicts, we might send pull requests + to other trees (e.g. tracing) with a small subset of the patches, but + net and net-next are always the main trees targeted for integration. + + The pull requests will contain a high-level summary of the accumulated + patches and can be searched on netdev kernel mailing list through the + following subject lines (yyyy-mm-dd is the date of the pull request): + + pull-request: bpf yyyy-mm-dd + pull-request: bpf-next yyyy-mm-dd + +Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be + applied to? + +A: The process is the very same as described in the netdev FAQ, so + please read up on it. The subject line must indicate whether the + patch is a fix or rather "next-like" content in order to let the + maintainers know whether it is targeted at bpf or bpf-next. + + For fixes eventually landing in bpf -> net tree, the subject must + look like: + + git format-patch --subject-prefix='PATCH bpf' start..finish + + For features/improvements/etc that should eventually land in + bpf-next -> net-next, the subject must look like: + + git format-patch --subject-prefix='PATCH bpf-next' start..finish + + If unsure whether the patch or patch series should go into bpf + or net directly, or bpf-next or net-next directly, it is not a + problem either if the subject line says net or net-next as target. + It is eventually up to the maintainers to do the delegation of + the patches. + + If it is clear that patches should go into bpf or bpf-next tree, + please make sure to rebase the patches against those trees in + order to reduce potential conflicts. + + In case the patch or patch series has to be reworked and sent out + again in a second or later revision, it is also required to add a + version number (v2, v3, ...) into the subject prefix: + + git format-patch --subject-prefix='PATCH net-next v2' start..finish + + When changes have been requested to the patch series, always send the + whole patch series again with the feedback incorporated (never send + individual diffs on top of the old series). + +Q: What does it mean when a patch gets applied to bpf or bpf-next tree? + +A: It means that the patch looks good for mainline inclusion from + a BPF point of view. + + Be aware that this is not a final verdict that the patch will + automatically get accepted into net or net-next trees eventually: + + On the netdev kernel mailing list reviews can come in at any point + in time. If discussions around a patch conclude that they cannot + get included as-is, we will either apply a follow-up fix or drop + them from the trees entirely. Therefore, we also reserve to rebase + the trees when deemed necessary. After all, the purpose of the tree + is to i) accumulate and stage BPF patches for integration into trees + like net and net-next, and ii) run extensive BPF test suite and + workloads on the patches before they make their way any further. + + Once the BPF pull request was accepted by David S. Miller, then + the patches end up in net or net-next tree, respectively, and + make their way from there further into mainline. Again, see the + netdev FAQ for additional information e.g. on how often they are + merged to mainline. + +Q: How long do I need to wait for feedback on my BPF patches? + +A: We try to keep the latency low. The usual time to feedback will + be around 2 or 3 business days. It may vary depending on the + complexity of changes and current patch load. + +Q: How often do you send pull requests to major kernel trees like + net or net-next? + +A: Pull requests will be sent out rather often in order to not + accumulate too many patches in bpf or bpf-next. + + As a rule of thumb, expect pull requests for each tree regularly + at the end of the week. In some cases pull requests could additionally + come also in the middle of the week depending on the current patch + load or urgency. + +Q: Are patches applied to bpf-next when the merge window is open? + +A: For the time when the merge window is open, bpf-next will not be + processed. This is roughly analogous to net-next patch processing, + so feel free to read up on the netdev FAQ about further details. + + During those two weeks of merge window, we might ask you to resend + your patch series once bpf-next is open again. Once Linus released + a v*-rc1 after the merge window, we continue processing of bpf-next. + + For non-subscribers to kernel mailing lists, there is also a status + page run by David S. Miller on net-next that provides guidance: + + http://vger.kernel.org/~davem/net-next.html + +Q: I made a BPF verifier change, do I need to add test cases for + BPF kernel selftests? + +A: If the patch has changes to the behavior of the verifier, then yes, + it is absolutely necessary to add test cases to the BPF kernel + selftests suite. If they are not present and we think they are + needed, then we might ask for them before accepting any changes. + + In particular, test_verifier.c is tracking a high number of BPF test + cases, including a lot of corner cases that LLVM BPF back end may + generate out of the restricted C code. Thus, adding test cases is + absolutely crucial to make sure future changes do not accidentally + affect prior use-cases. Thus, treat those test cases as: verifier + behavior that is not tracked in test_verifier.c could potentially + be subject to change. + +Q: When should I add code to samples/bpf/ and when to BPF kernel + selftests? + +A: In general, we prefer additions to BPF kernel selftests rather than + samples/bpf/. The rationale is very simple: kernel selftests are + regularly run by various bots to test for kernel regressions. + + The more test cases we add to BPF selftests, the better the coverage + and the less likely it is that those could accidentally break. It is + not that BPF kernel selftests cannot demo how a specific feature can + be used. + + That said, samples/bpf/ may be a good place for people to get started, + so it might be advisable that simple demos of features could go into + samples/bpf/, but advanced functional and corner-case testing rather + into kernel selftests. + + If your sample looks like a test case, then go for BPF kernel selftests + instead! + +Q: When should I add code to the bpftool? + +A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide + a central user space tool for debugging and introspection of BPF programs + and maps that are active in the kernel. If UAPI changes related to BPF + enable for dumping additional information of programs or maps, then + bpftool should be extended as well to support dumping them. + +Q: When should I add code to iproute2's BPF loader? + +A: For UAPI changes related to the XDP or tc layer (e.g. cls_bpf), the + convention is that those control-path related changes are added to + iproute2's BPF loader as well from user space side. This is not only + useful to have UAPI changes properly designed to be usable, but also + to make those changes available to a wider user base of major + downstream distributions. + +Q: Do you accept patches as well for iproute2's BPF loader? + +A: Patches for the iproute2's BPF loader have to be sent to: + + netdev@xxxxxxxxxxxxxxx + + While those patches are not processed by the BPF kernel maintainers, + please keep them in Cc as well, so they can be reviewed. + + The official git repository for iproute2 is run by Stephen Hemminger + and can be found at: + + https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/ + + The patches need to have a subject prefix of '[PATCH iproute2 master]' + or '[PATCH iproute2 net-next]'. 'master' or 'net-next' describes the + target branch where the patch should be applied to. Meaning, if kernel + changes went into the net-next kernel tree, then the related iproute2 + changes need to go into the iproute2 net-next branch, otherwise they + can be targeted at master branch. The iproute2 net-next branch will get + merged into the master branch after the current iproute2 version from + master has been released. + + Like BPF, the patches end up in patchwork under the netdev project and + are delegated to 'shemminger' for further processing: + + http://patchwork.ozlabs.org/project/netdev/list/?delegate=389 + +Q: What is the minimum requirement before I submit my BPF patches? + +A: When submitting patches, always take the time and properly test your + patches *prior* to submission. Never rush them! If maintainers find + that your patches have not been properly tested, it is a good way to + get them grumpy. Testing patch submissions is a hard requirement! + + Note, fixes that go to bpf tree *must* have a Fixes: tag included. The + same applies to fixes that target bpf-next, where the affected commit + is in net-next (or in some cases bpf-next). The Fixes: tag is crucial + in order to identify follow-up commits and tremendously helps for people + having to do backporting, so it is a must have! + + We also don't accept patches with an empty commit message. Take your + time and properly write up a high quality commit message, it is + essential! + + Think about it this way: other developers looking at your code a month + from now need to understand *why* a certain change has been done that + way, and whether there have been flaws in the analysis or assumptions + that the original author did. Thus providing a proper rationale and + describing the use-case for the changes is a must. + + Patch submissions with >1 patch must have a cover letter which includes + a high level description of the series. This high level summary will + then be placed into the merge commit by the BPF maintainers such that + it is also accessible from the git log for future reference. + +Q: What do I need to consider when adding a new instruction or feature + that would require BPF JIT and/or LLVM integration as well? + +A: We try hard to keep all BPF JITs up to date such that the same user + experience can be guaranteed when running BPF programs on different + architectures without having the program punt to the less efficient + interpreter in case the in-kernel BPF JIT is enabled. + + If you are unable to implement or test the required JIT changes for + certain architectures, please work together with the related BPF JIT + developers in order to get the feature implemented in a timely manner. + Please refer to the git log (arch/*/net/) to locate the necessary + people for helping out. + + Also always make sure to add BPF test cases (e.g. test_bpf.c and + test_verifier.c) for new instructions, so that they can receive + broad test coverage and help run-time testing the various BPF JITs. + + In case of new BPF instructions, once the changes have been accepted + into the Linux kernel, please implement support into LLVM's BPF back + end. See LLVM section below for further information. + +Stable submission: +------------------ + +Q: I need a specific BPF commit in stable kernels. What should I do? + +A: In case you need a specific fix in stable kernels, first check whether + the commit has already been applied in the related linux-*.y branches: + + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/ + + If not the case, then drop an email to the BPF maintainers with the + netdev kernel mailing list in Cc and ask for the fix to be queued up: + + netdev@xxxxxxxxxxxxxxx + + The process in general is the same as on netdev itself, see also the + netdev FAQ document. + +Q: Do you also backport to kernels not currently maintained as stable? + +A: No. If you need a specific BPF commit in kernels that are currently not + maintained by the stable maintainers, then you are on your own. + + The current stable and longterm stable kernels are all listed here: + + https://www.kernel.org/ + +Q: The BPF patch I am about to submit needs to go to stable as well. What + should I do? + +A: The same rules apply as with netdev patch submissions in general, see + netdev FAQ under: + + Documentation/networking/netdev-FAQ.txt + + Never add "Cc: stable@xxxxxxxxxxxxxxx" to the patch description, but + ask the BPF maintainers to queue the patches instead. This can be done + with a note, for example, under the "---" part of the patch which does + not go into the git log. Alternatively, this can be done as a simple + request by mail instead. + +Q: Where do I find currently queued BPF patches that will be submitted + to stable? + +A: Once patches that fix critical bugs got applied into the bpf tree, they + are queued up for stable submission under: + + http://patchwork.ozlabs.org/bundle/bpf/stable/?state=* + + They will be on hold there at minimum until the related commit made its + way into the mainline kernel tree. + + After having been under broader exposure, the queued patches will be + submitted by the BPF maintainers to the stable maintainers. + +Testing patches: +---------------- + +Q: Which BPF kernel selftests version should I run my kernel against? + +A: If you run a kernel xyz, then always run the BPF kernel selftests from + that kernel xyz as well. Do not expect that the BPF selftest from the + latest mainline tree will pass all the time. + + In particular, test_bpf.c and test_verifier.c have a large number of + test cases and are constantly updated with new BPF test sequences, or + existing ones are adapted to verifier changes e.g. due to verifier + becoming smarter and being able to better track certain things. + +LLVM: +----- + +Q: Where do I find LLVM with BPF support? + +A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1. + + All major distributions these days ship LLVM with BPF back end enabled, + so for the majority of use-cases it is not required to compile LLVM by + hand anymore, just install the distribution provided package. + + LLVM's static compiler lists the supported targets through 'llc --version', + make sure BPF targets are listed. Example: + + $ llc --version + LLVM (http://llvm.org/): + LLVM version 6.0.0svn + Optimized build. + Default target: x86_64-unknown-linux-gnu + Host CPU: skylake + + Registered Targets: + bpf - BPF (host endian) + bpfeb - BPF (big endian) + bpfel - BPF (little endian) + x86 - 32-bit X86: Pentium-Pro and above + x86-64 - 64-bit X86: EM64T and AMD64 + + For developers in order to utilize the latest features added to LLVM's + BPF back end, it is advisable to run the latest LLVM releases. Support + for new BPF kernel features such as additions to the BPF instruction + set are often developed together. + + All LLVM releases can be found at: http://releases.llvm.org/ + +Q: Got it, so how do I build LLVM manually anyway? + +A: You need cmake and gcc-c++ as build requisites for LLVM. Once you have + that set up, proceed with building the latest LLVM and clang version + from the git repositories: + + $ git clone http://llvm.org/git/llvm.git + $ cd llvm/tools + $ git clone --depth 1 http://llvm.org/git/clang.git + $ cd ..; mkdir build; cd build + $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86" \ + -DBUILD_SHARED_LIBS=OFF \ + -DCMAKE_BUILD_TYPE=Release \ + -DLLVM_BUILD_RUNTIME=OFF + $ make -j $(getconf _NPROCESSORS_ONLN) + + The built binaries can then be found in the build/bin/ directory, where + you can point the PATH variable to. + +Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code + generation back end or about LLVM generated code that the verifier + refuses to accept? + +A: Yes, please do! LLVM's BPF back end is a key piece of the whole BPF + infrastructure and it ties deeply into verification of programs from the + kernel side. Therefore, any issues on either side need to be investigated + and fixed whenever necessary. + + Therefore, please make sure to bring them up at netdev kernel mailing + list and Cc BPF maintainers for LLVM and kernel bits: + + Yonghong Song <yhs@xxxxxx> + Alexei Starovoitov <ast@xxxxxxxxxx> + Daniel Borkmann <daniel@xxxxxxxxxxxxx> + + LLVM also has an issue tracker where BPF related bugs can be found: + + https://bugs.llvm.org/buglist.cgi?quicksearch=bpf + + However, it is better to reach out through mailing lists with having + maintainers in Cc. + +Q: I have added a new BPF instruction to the kernel, how can I integrate + it into LLVM? + +A: LLVM has a -mcpu selector for the BPF back end in order to allow the + selection of BPF instruction set extensions. By default the 'generic' + processor target is used, which is the base instruction set (v1) of BPF. + + LLVM has an option to select -mcpu=probe where it will probe the host + kernel for supported BPF instruction set extensions and selects the + optimal set automatically. + + For cross-compilation, a specific version can be select manually as well. + + $ llc -march bpf -mcpu=help + Available CPUs for this target: + + generic - Select the generic processor. + probe - Select the probe processor. + v1 - Select the v1 processor. + v2 - Select the v2 processor. + [...] + + Newly added BPF instructions to the Linux kernel need to follow the same + scheme, bump the instruction set version and implement probing for the + extensions such that -mcpu=probe users can benefit from the optimization + transparently when upgrading their kernels. + + If you are unable to implement support for the newly added BPF instruction + please reach out to BPF developers for help. + + By the way, the BPF kernel selftests run with -mcpu=probe for better + test coverage. + +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: + + - 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 + understand these assembly codes, which is true in most cases. + + - 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. + + - 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 + switch table generation. + + - 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 + compile these types based on the underlying architecture's conventions, + 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 + or other kernel structures where CPU's register width matters. + 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 + when: + + - Your program uses data structures with pointer or long / unsigned long + types that interface with BPF helpers or context data structures. Access + 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' + +Happy BPF hacking! diff --git a/Documentation/bpf/bpf_devel_QA.txt b/Documentation/bpf/bpf_devel_QA.txt deleted file mode 100644 index da57601153a0..000000000000 --- a/Documentation/bpf/bpf_devel_QA.txt +++ /dev/null @@ -1,570 +0,0 @@ -This document provides information for the BPF subsystem about various -workflows related to reporting bugs, submitting patches, and queueing -patches for stable kernels. - -For general information about submitting patches, please refer to -Documentation/process/. This document only describes additional specifics -related to BPF. - -Reporting bugs: ---------------- - -Q: How do I report bugs for BPF kernel code? - -A: Since all BPF kernel development as well as bpftool and iproute2 BPF - loader development happens through the netdev kernel mailing list, - please report any found issues around BPF to the following mailing - list: - - netdev@xxxxxxxxxxxxxxx - - This may also include issues related to XDP, BPF tracing, etc. - - Given netdev has a high volume of traffic, please also add the BPF - maintainers to Cc (from kernel MAINTAINERS file): - - Alexei Starovoitov <ast@xxxxxxxxxx> - Daniel Borkmann <daniel@xxxxxxxxxxxxx> - - In case a buggy commit has already been identified, make sure to keep - the actual commit authors in Cc as well for the report. They can - typically be identified through the kernel's git tree. - - Please do *not* report BPF issues to bugzilla.kernel.org since it - is a guarantee that the reported issue will be overlooked. - -Submitting patches: -------------------- - -Q: To which mailing list do I need to submit my BPF patches? - -A: Please submit your BPF patches to the netdev kernel mailing list: - - netdev@xxxxxxxxxxxxxxx - - Historically, BPF came out of networking and has always been maintained - by the kernel networking community. Although these days BPF touches - many other subsystems as well, the patches are still routed mainly - through the networking community. - - In case your patch has changes in various different subsystems (e.g. - tracing, security, etc), make sure to Cc the related kernel mailing - lists and maintainers from there as well, so they are able to review - the changes and provide their Acked-by's to the patches. - -Q: Where can I find patches currently under discussion for BPF subsystem? - -A: All patches that are Cc'ed to netdev are queued for review under netdev - patchwork project: - - http://patchwork.ozlabs.org/project/netdev/list/ - - Those patches which target BPF, are assigned to a 'bpf' delegate for - further processing from BPF maintainers. The current queue with - patches under review can be found at: - - https://patchwork.ozlabs.org/project/netdev/list/?delegate=77147 - - Once the patches have been reviewed by the BPF community as a whole - and approved by the BPF maintainers, their status in patchwork will be - changed to 'Accepted' and the submitter will be notified by mail. This - means that the patches look good from a BPF perspective and have been - applied to one of the two BPF kernel trees. - - In case feedback from the community requires a respin of the patches, - their status in patchwork will be set to 'Changes Requested', and purged - from the current review queue. Likewise for cases where patches would - get rejected or are not applicable to the BPF trees (but assigned to - the 'bpf' delegate). - -Q: How do the changes make their way into Linux? - -A: There are two BPF kernel trees (git repositories). Once patches have - been accepted by the BPF maintainers, they will be applied to one - of the two BPF trees: - - https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/ - https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/ - - The bpf tree itself is for fixes only, whereas bpf-next for features, - cleanups or other kind of improvements ("next-like" content). This is - analogous to net and net-next trees for networking. Both bpf and - bpf-next will only have a master branch in order to simplify against - which branch patches should get rebased to. - - Accumulated BPF patches in the bpf tree will regularly get pulled - into the net kernel tree. Likewise, accumulated BPF patches accepted - into the bpf-next tree will make their way into net-next tree. net and - net-next are both run by David S. Miller. From there, they will go - into the kernel mainline tree run by Linus Torvalds. To read up on the - process of net and net-next being merged into the mainline tree, see - the netdev FAQ under: - - Documentation/networking/netdev-FAQ.txt - - Occasionally, to prevent merge conflicts, we might send pull requests - to other trees (e.g. tracing) with a small subset of the patches, but - net and net-next are always the main trees targeted for integration. - - The pull requests will contain a high-level summary of the accumulated - patches and can be searched on netdev kernel mailing list through the - following subject lines (yyyy-mm-dd is the date of the pull request): - - pull-request: bpf yyyy-mm-dd - pull-request: bpf-next yyyy-mm-dd - -Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be - applied to? - -A: The process is the very same as described in the netdev FAQ, so - please read up on it. The subject line must indicate whether the - patch is a fix or rather "next-like" content in order to let the - maintainers know whether it is targeted at bpf or bpf-next. - - For fixes eventually landing in bpf -> net tree, the subject must - look like: - - git format-patch --subject-prefix='PATCH bpf' start..finish - - For features/improvements/etc that should eventually land in - bpf-next -> net-next, the subject must look like: - - git format-patch --subject-prefix='PATCH bpf-next' start..finish - - If unsure whether the patch or patch series should go into bpf - or net directly, or bpf-next or net-next directly, it is not a - problem either if the subject line says net or net-next as target. - It is eventually up to the maintainers to do the delegation of - the patches. - - If it is clear that patches should go into bpf or bpf-next tree, - please make sure to rebase the patches against those trees in - order to reduce potential conflicts. - - In case the patch or patch series has to be reworked and sent out - again in a second or later revision, it is also required to add a - version number (v2, v3, ...) into the subject prefix: - - git format-patch --subject-prefix='PATCH net-next v2' start..finish - - When changes have been requested to the patch series, always send the - whole patch series again with the feedback incorporated (never send - individual diffs on top of the old series). - -Q: What does it mean when a patch gets applied to bpf or bpf-next tree? - -A: It means that the patch looks good for mainline inclusion from - a BPF point of view. - - Be aware that this is not a final verdict that the patch will - automatically get accepted into net or net-next trees eventually: - - On the netdev kernel mailing list reviews can come in at any point - in time. If discussions around a patch conclude that they cannot - get included as-is, we will either apply a follow-up fix or drop - them from the trees entirely. Therefore, we also reserve to rebase - the trees when deemed necessary. After all, the purpose of the tree - is to i) accumulate and stage BPF patches for integration into trees - like net and net-next, and ii) run extensive BPF test suite and - workloads on the patches before they make their way any further. - - Once the BPF pull request was accepted by David S. Miller, then - the patches end up in net or net-next tree, respectively, and - make their way from there further into mainline. Again, see the - netdev FAQ for additional information e.g. on how often they are - merged to mainline. - -Q: How long do I need to wait for feedback on my BPF patches? - -A: We try to keep the latency low. The usual time to feedback will - be around 2 or 3 business days. It may vary depending on the - complexity of changes and current patch load. - -Q: How often do you send pull requests to major kernel trees like - net or net-next? - -A: Pull requests will be sent out rather often in order to not - accumulate too many patches in bpf or bpf-next. - - As a rule of thumb, expect pull requests for each tree regularly - at the end of the week. In some cases pull requests could additionally - come also in the middle of the week depending on the current patch - load or urgency. - -Q: Are patches applied to bpf-next when the merge window is open? - -A: For the time when the merge window is open, bpf-next will not be - processed. This is roughly analogous to net-next patch processing, - so feel free to read up on the netdev FAQ about further details. - - During those two weeks of merge window, we might ask you to resend - your patch series once bpf-next is open again. Once Linus released - a v*-rc1 after the merge window, we continue processing of bpf-next. - - For non-subscribers to kernel mailing lists, there is also a status - page run by David S. Miller on net-next that provides guidance: - - http://vger.kernel.org/~davem/net-next.html - -Q: I made a BPF verifier change, do I need to add test cases for - BPF kernel selftests? - -A: If the patch has changes to the behavior of the verifier, then yes, - it is absolutely necessary to add test cases to the BPF kernel - selftests suite. If they are not present and we think they are - needed, then we might ask for them before accepting any changes. - - In particular, test_verifier.c is tracking a high number of BPF test - cases, including a lot of corner cases that LLVM BPF back end may - generate out of the restricted C code. Thus, adding test cases is - absolutely crucial to make sure future changes do not accidentally - affect prior use-cases. Thus, treat those test cases as: verifier - behavior that is not tracked in test_verifier.c could potentially - be subject to change. - -Q: When should I add code to samples/bpf/ and when to BPF kernel - selftests? - -A: In general, we prefer additions to BPF kernel selftests rather than - samples/bpf/. The rationale is very simple: kernel selftests are - regularly run by various bots to test for kernel regressions. - - The more test cases we add to BPF selftests, the better the coverage - and the less likely it is that those could accidentally break. It is - not that BPF kernel selftests cannot demo how a specific feature can - be used. - - That said, samples/bpf/ may be a good place for people to get started, - so it might be advisable that simple demos of features could go into - samples/bpf/, but advanced functional and corner-case testing rather - into kernel selftests. - - If your sample looks like a test case, then go for BPF kernel selftests - instead! - -Q: When should I add code to the bpftool? - -A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide - a central user space tool for debugging and introspection of BPF programs - and maps that are active in the kernel. If UAPI changes related to BPF - enable for dumping additional information of programs or maps, then - bpftool should be extended as well to support dumping them. - -Q: When should I add code to iproute2's BPF loader? - -A: For UAPI changes related to the XDP or tc layer (e.g. cls_bpf), the - convention is that those control-path related changes are added to - iproute2's BPF loader as well from user space side. This is not only - useful to have UAPI changes properly designed to be usable, but also - to make those changes available to a wider user base of major - downstream distributions. - -Q: Do you accept patches as well for iproute2's BPF loader? - -A: Patches for the iproute2's BPF loader have to be sent to: - - netdev@xxxxxxxxxxxxxxx - - While those patches are not processed by the BPF kernel maintainers, - please keep them in Cc as well, so they can be reviewed. - - The official git repository for iproute2 is run by Stephen Hemminger - and can be found at: - - https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/ - - The patches need to have a subject prefix of '[PATCH iproute2 master]' - or '[PATCH iproute2 net-next]'. 'master' or 'net-next' describes the - target branch where the patch should be applied to. Meaning, if kernel - changes went into the net-next kernel tree, then the related iproute2 - changes need to go into the iproute2 net-next branch, otherwise they - can be targeted at master branch. The iproute2 net-next branch will get - merged into the master branch after the current iproute2 version from - master has been released. - - Like BPF, the patches end up in patchwork under the netdev project and - are delegated to 'shemminger' for further processing: - - http://patchwork.ozlabs.org/project/netdev/list/?delegate=389 - -Q: What is the minimum requirement before I submit my BPF patches? - -A: When submitting patches, always take the time and properly test your - patches *prior* to submission. Never rush them! If maintainers find - that your patches have not been properly tested, it is a good way to - get them grumpy. Testing patch submissions is a hard requirement! - - Note, fixes that go to bpf tree *must* have a Fixes: tag included. The - same applies to fixes that target bpf-next, where the affected commit - is in net-next (or in some cases bpf-next). The Fixes: tag is crucial - in order to identify follow-up commits and tremendously helps for people - having to do backporting, so it is a must have! - - We also don't accept patches with an empty commit message. Take your - time and properly write up a high quality commit message, it is - essential! - - Think about it this way: other developers looking at your code a month - from now need to understand *why* a certain change has been done that - way, and whether there have been flaws in the analysis or assumptions - that the original author did. Thus providing a proper rationale and - describing the use-case for the changes is a must. - - Patch submissions with >1 patch must have a cover letter which includes - a high level description of the series. This high level summary will - then be placed into the merge commit by the BPF maintainers such that - it is also accessible from the git log for future reference. - -Q: What do I need to consider when adding a new instruction or feature - that would require BPF JIT and/or LLVM integration as well? - -A: We try hard to keep all BPF JITs up to date such that the same user - experience can be guaranteed when running BPF programs on different - architectures without having the program punt to the less efficient - interpreter in case the in-kernel BPF JIT is enabled. - - If you are unable to implement or test the required JIT changes for - certain architectures, please work together with the related BPF JIT - developers in order to get the feature implemented in a timely manner. - Please refer to the git log (arch/*/net/) to locate the necessary - people for helping out. - - Also always make sure to add BPF test cases (e.g. test_bpf.c and - test_verifier.c) for new instructions, so that they can receive - broad test coverage and help run-time testing the various BPF JITs. - - In case of new BPF instructions, once the changes have been accepted - into the Linux kernel, please implement support into LLVM's BPF back - end. See LLVM section below for further information. - -Stable submission: ------------------- - -Q: I need a specific BPF commit in stable kernels. What should I do? - -A: In case you need a specific fix in stable kernels, first check whether - the commit has already been applied in the related linux-*.y branches: - - https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/ - - If not the case, then drop an email to the BPF maintainers with the - netdev kernel mailing list in Cc and ask for the fix to be queued up: - - netdev@xxxxxxxxxxxxxxx - - The process in general is the same as on netdev itself, see also the - netdev FAQ document. - -Q: Do you also backport to kernels not currently maintained as stable? - -A: No. If you need a specific BPF commit in kernels that are currently not - maintained by the stable maintainers, then you are on your own. - - The current stable and longterm stable kernels are all listed here: - - https://www.kernel.org/ - -Q: The BPF patch I am about to submit needs to go to stable as well. What - should I do? - -A: The same rules apply as with netdev patch submissions in general, see - netdev FAQ under: - - Documentation/networking/netdev-FAQ.txt - - Never add "Cc: stable@xxxxxxxxxxxxxxx" to the patch description, but - ask the BPF maintainers to queue the patches instead. This can be done - with a note, for example, under the "---" part of the patch which does - not go into the git log. Alternatively, this can be done as a simple - request by mail instead. - -Q: Where do I find currently queued BPF patches that will be submitted - to stable? - -A: Once patches that fix critical bugs got applied into the bpf tree, they - are queued up for stable submission under: - - http://patchwork.ozlabs.org/bundle/bpf/stable/?state=* - - They will be on hold there at minimum until the related commit made its - way into the mainline kernel tree. - - After having been under broader exposure, the queued patches will be - submitted by the BPF maintainers to the stable maintainers. - -Testing patches: ----------------- - -Q: Which BPF kernel selftests version should I run my kernel against? - -A: If you run a kernel xyz, then always run the BPF kernel selftests from - that kernel xyz as well. Do not expect that the BPF selftest from the - latest mainline tree will pass all the time. - - In particular, test_bpf.c and test_verifier.c have a large number of - test cases and are constantly updated with new BPF test sequences, or - existing ones are adapted to verifier changes e.g. due to verifier - becoming smarter and being able to better track certain things. - -LLVM: ------ - -Q: Where do I find LLVM with BPF support? - -A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1. - - All major distributions these days ship LLVM with BPF back end enabled, - so for the majority of use-cases it is not required to compile LLVM by - hand anymore, just install the distribution provided package. - - LLVM's static compiler lists the supported targets through 'llc --version', - make sure BPF targets are listed. Example: - - $ llc --version - LLVM (http://llvm.org/): - LLVM version 6.0.0svn - Optimized build. - Default target: x86_64-unknown-linux-gnu - Host CPU: skylake - - Registered Targets: - bpf - BPF (host endian) - bpfeb - BPF (big endian) - bpfel - BPF (little endian) - x86 - 32-bit X86: Pentium-Pro and above - x86-64 - 64-bit X86: EM64T and AMD64 - - For developers in order to utilize the latest features added to LLVM's - BPF back end, it is advisable to run the latest LLVM releases. Support - for new BPF kernel features such as additions to the BPF instruction - set are often developed together. - - All LLVM releases can be found at: http://releases.llvm.org/ - -Q: Got it, so how do I build LLVM manually anyway? - -A: You need cmake and gcc-c++ as build requisites for LLVM. Once you have - that set up, proceed with building the latest LLVM and clang version - from the git repositories: - - $ git clone http://llvm.org/git/llvm.git - $ cd llvm/tools - $ git clone --depth 1 http://llvm.org/git/clang.git - $ cd ..; mkdir build; cd build - $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86" \ - -DBUILD_SHARED_LIBS=OFF \ - -DCMAKE_BUILD_TYPE=Release \ - -DLLVM_BUILD_RUNTIME=OFF - $ make -j $(getconf _NPROCESSORS_ONLN) - - The built binaries can then be found in the build/bin/ directory, where - you can point the PATH variable to. - -Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code - generation back end or about LLVM generated code that the verifier - refuses to accept? - -A: Yes, please do! LLVM's BPF back end is a key piece of the whole BPF - infrastructure and it ties deeply into verification of programs from the - kernel side. Therefore, any issues on either side need to be investigated - and fixed whenever necessary. - - Therefore, please make sure to bring them up at netdev kernel mailing - list and Cc BPF maintainers for LLVM and kernel bits: - - Yonghong Song <yhs@xxxxxx> - Alexei Starovoitov <ast@xxxxxxxxxx> - Daniel Borkmann <daniel@xxxxxxxxxxxxx> - - LLVM also has an issue tracker where BPF related bugs can be found: - - https://bugs.llvm.org/buglist.cgi?quicksearch=bpf - - However, it is better to reach out through mailing lists with having - maintainers in Cc. - -Q: I have added a new BPF instruction to the kernel, how can I integrate - it into LLVM? - -A: LLVM has a -mcpu selector for the BPF back end in order to allow the - selection of BPF instruction set extensions. By default the 'generic' - processor target is used, which is the base instruction set (v1) of BPF. - - LLVM has an option to select -mcpu=probe where it will probe the host - kernel for supported BPF instruction set extensions and selects the - optimal set automatically. - - For cross-compilation, a specific version can be select manually as well. - - $ llc -march bpf -mcpu=help - Available CPUs for this target: - - generic - Select the generic processor. - probe - Select the probe processor. - v1 - Select the v1 processor. - v2 - Select the v2 processor. - [...] - - Newly added BPF instructions to the Linux kernel need to follow the same - scheme, bump the instruction set version and implement probing for the - extensions such that -mcpu=probe users can benefit from the optimization - transparently when upgrading their kernels. - - If you are unable to implement support for the newly added BPF instruction - please reach out to BPF developers for help. - - By the way, the BPF kernel selftests run with -mcpu=probe for better - test coverage. - -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: - - - 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 - understand these assembly codes, which is true in most cases. - - - 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. - - - 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 - switch table generation. - - - 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 - compile these types based on the underlying architecture's conventions, - 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 - or other kernel structures where CPU's register width matters. - 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 - when: - - - Your program uses data structures with pointer or long / unsigned long - types that interface with BPF helpers or context data structures. Access - 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' - -Happy BPF hacking! -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
