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authorDaniel Borkmann <daniel@iogearbox.net>2018-03-20 00:21:15 +0100
committerAlexei Starovoitov <ast@kernel.org>2018-03-20 15:47:45 -0700
commit78262f4575c29f185947fe58952cd1beabc74f82 (patch)
treed25461aba3c6fbddcfc3a2cfaa582de422893f8f /Documentation/bpf
parentd48ce3e5ba741428ed38a665a3c6b41e6cd999be (diff)
bpf, doc: add description wrt native/bpf clang target and pointer size
As this recently came up on netdev [0], lets add it to the BPF devel doc. [0] https://www.spinics.net/lists/netdev/msg489612.html Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Diffstat (limited to 'Documentation/bpf')
-rw-r--r--Documentation/bpf/bpf_devel_QA.txt12
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