summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorJosh Poimboeuf <jpoimboe@redhat.com>2019-07-17 20:36:45 -0500
committerThomas Gleixner <tglx@linutronix.de>2019-07-18 21:01:06 +0200
commit3193c0836f203a91bef96d88c64cccf0be090d9c (patch)
tree4247cde8f412fc53e496c24ee91e5c1bd32dbd96
parent82e844a6536d1a3c12a73e44712f4021d90a4b53 (diff)
bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()
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.1563413318.git.jpoimboe@redhat.com
-rw-r--r--include/linux/compiler-gcc.h2
-rw-r--r--include/linux/compiler_types.h4
-rw-r--r--kernel/bpf/core.c2
3 files changed, 7 insertions, 1 deletions
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 095d55c3834d..599c27b56c29 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -189,6 +189,10 @@ 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 7e98f36a14e2..8191a7db2777 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