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Date: Thu, 1 Feb 2018 23:00:11 -0800
From: Yonghong Song <yhs@...com>
To: <ast@...com>, <daniel@...earbox.net>, <netdev@...r.kernel.org>
CC: <kernel-team@...com>
Subject: [PATCH bpf-next] 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: Yonghong Song <yhs@...com>
---
Documentation/bpf/bpf_devel_QA.txt | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)
diff --git a/Documentation/bpf/bpf_devel_QA.txt b/Documentation/bpf/bpf_devel_QA.txt
index cefef85..3a3e77c 100644
--- a/Documentation/bpf/bpf_devel_QA.txt
+++ b/Documentation/bpf/bpf_devel_QA.txt
@@ -516,4 +516,34 @@ 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
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?
+
+ 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. 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!
--
2.9.5
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