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author | 2019-07-17 20:36:45 -0500 | |
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committer | 2019-07-18 21:01:06 +0200 | |
commit | 3193c0836f203a91bef96d88c64cccf0be090d9c (patch) | |
tree | 4247cde8f412fc53e496c24ee91e5c1bd32dbd96 /include | |
parent | x86/uaccess: Remove redundant CLACs in getuser/putuser error paths (diff) | |
download | linux-3193c0836f203a91bef96d88c64cccf0be090d9c.tar.gz linux-3193c0836f203a91bef96d88c64cccf0be090d9c.tar.bz2 linux-3193c0836f203a91bef96d88c64cccf0be090d9c.zip |
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
Diffstat (limited to 'include')
-rw-r--r-- | include/linux/compiler-gcc.h | 2 | ||||
-rw-r--r-- | include/linux/compiler_types.h | 4 |
2 files changed, 6 insertions, 0 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)) |