Commit 6215ea6b authored by Yonghong Song's avatar Yonghong Song Committed by Daniel Borkmann

bpf: add documentation to compare clang "-target bpf" and default target

The added documentation explains how generated codes may differ
between clang bpf target and default target, and when to use
each target.
Signed-off-by: default avatarYonghong Song <yhs@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent 65073a67
...@@ -516,4 +516,35 @@ A: LLVM has a -mcpu selector for the BPF back end in order to allow the ...@@ -516,4 +516,35 @@ A: LLVM has a -mcpu selector for the BPF back end in order to allow the
By the way, the BPF kernel selftests run with -mcpu=probe for better By the way, the BPF kernel selftests run with -mcpu=probe for better
test coverage. 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.
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.
Happy BPF hacking! Happy BPF hacking!
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