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Diffstat (limited to 'Documentation/bpf/bpf_devel_QA.rst')
| -rw-r--r-- | Documentation/bpf/bpf_devel_QA.rst | 25 |
1 files changed, 17 insertions, 8 deletions
diff --git a/Documentation/bpf/bpf_devel_QA.rst b/Documentation/bpf/bpf_devel_QA.rst index 609b71f5747d..45bc5c5cd793 100644 --- a/Documentation/bpf/bpf_devel_QA.rst +++ b/Documentation/bpf/bpf_devel_QA.rst @@ -382,6 +382,14 @@ 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. +Q: What "BPF_INTERNAL" symbol namespace is for? +----------------------------------------------- +A: Symbols exported as BPF_INTERNAL can only be used by BPF infrastructure +like preload kernel modules with light skeleton. Most symbols outside +of BPF_INTERNAL are not expected to be used by code outside of BPF either. +Symbols may lack the designation because they predate the namespaces, +or due to an oversight. + Stable submission ================= @@ -603,9 +611,10 @@ 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. +the selection of BPF instruction set extensions. Before llvm version 20, +the ``generic`` processor target is used, which is the base instruction +set (v1) of BPF. Since llvm 20, the default processor target has changed +to instruction set v3. LLVM has an option to select ``-mcpu=probe`` where it will probe the host kernel for supported BPF instruction set extensions and selects the @@ -635,12 +644,12 @@ test coverage. Q: clang flag for target bpf? ----------------------------- -Q: In some cases clang flag ``-target bpf`` is used but in other cases the +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, @@ -658,7 +667,7 @@ independent, ``-target <arch>`` still has some impact on generated code: 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 @@ -668,7 +677,7 @@ independent, ``-target <arch>`` still has some impact on generated code: 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. + Otherwise, ``clang --target=bpf`` is generally recommended. You should use default target when: @@ -685,7 +694,7 @@ when: 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 |