From: Josh Poimboeuf jpoimboe@redhat.com
[ Upstream commit 3193c0836f203a91bef96d88c64cccf0be090d9c ]
On x86-64, with CONFIG_RETPOLINE=n, GCC's "global common subexpression elimination" optimization results in ___bpf_prog_run()'s jumptable code changing from this:
select_insn: jmp *jumptable(, %rax, 8) ... ALU64_ADD_X: ... jmp *jumptable(, %rax, 8) ALU_ADD_X: ... jmp *jumptable(, %rax, 8)
to this:
select_insn: mov jumptable, %r12 jmp *(%r12, %rax, 8) ... ALU64_ADD_X: ... jmp *(%r12, %rax, 8) ALU_ADD_X: ... jmp *(%r12, %rax, 8)
The jumptable address is placed in a register once, at the beginning of the function. The function execution can then go through multiple indirect jumps which rely on that same register value. This has a few issues:
1) Objtool isn't smart enough to be able to track such a register value across multiple recursive indirect jumps through the jump table.
2) With CONFIG_RETPOLINE enabled, this optimization actually results in a small slowdown. I measured a ~4.7% slowdown in the test_bpf "tcpdump port 22" selftest.
This slowdown is actually predicted by the GCC manual:
Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
So just disable the optimization for this function.
Fixes: e55a73251da3 ("bpf: Fix ORC unwinding in non-JIT BPF code") Reported-by: Randy Dunlap rdunlap@infradead.org Signed-off-by: Josh Poimboeuf jpoimboe@redhat.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Acked-by: Alexei Starovoitov ast@kernel.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lkml.kernel.org/r/30c3ca29ba037afcbd860a8672eef0021addf9fe.156341331... Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/compiler-gcc.h | 2 ++ include/linux/compiler_types.h | 4 ++++ kernel/bpf/core.c | 2 +- 3 files changed, 7 insertions(+), 1 deletion(-)
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h index e8579412ad21..d7ee4c6bad48 100644 --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h @@ -170,3 +170,5 @@ #else #define __diag_GCC_8(s) #endif + +#define __no_fgcse __attribute__((optimize("-fno-gcse"))) diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 19e58b9138a0..0454d82f8bd8 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -187,6 +187,10 @@ struct ftrace_likely_data { #define asm_volatile_goto(x...) asm goto(x) #endif
+#ifndef __no_fgcse +# define __no_fgcse +#endif + /* Are two types/vars the same type (ignoring qualifiers)? */ #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c index 080e2bb644cc..ebfd189916dc 100644 --- a/kernel/bpf/core.c +++ b/kernel/bpf/core.c @@ -1295,7 +1295,7 @@ bool bpf_opcode_in_insntable(u8 code) * * Decode and execute eBPF instructions. */ -static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) +static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) { #define BPF_INSN_2_LBL(x, y) [BPF_##x | BPF_##y] = &&x##_##y #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z