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| -rw-r--r-- | Documentation/bpf/bpf_devel_QA.txt | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/Documentation/bpf/bpf_devel_QA.txt b/Documentation/bpf/bpf_devel_QA.txt index 84cbb302f2b5..1a0b704e1a38 100644 --- a/Documentation/bpf/bpf_devel_QA.txt +++ b/Documentation/bpf/bpf_devel_QA.txt @@ -539,6 +539,18 @@ A: Although LLVM IR generation and optimization try to stay architecture 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 |