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 <dan...@iogearbox.net>
---
Documentation/bpf/bpf_devel_QA.txt | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/Documentation/bpf/bpf_devel_QA.txt
b/Documentation/bpf/bpf_devel_QA.txt
index 84cbb30..1a0b704 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
--
2.9.5
