"setbe %al" insn has a register merge stall: it needs to combine previous %eax value with new value for the lowest byte. Subsequent "movzbl %al,%edi" in turn depends on its completion.
This patch replaces "setbe %al + movzbl %al,%edi" pair of insns with "xor %edi,%edi" before the comparison, and conditional "inc %edi". This results in the same value of %edi as produced by old code, but first insn has no dependencies, and we end up with having only one insn with deps which executes only if %eax contains error return, and both insns are shorter: 2 bytes each versus 3 bytes each. (The old code was inherited from 32-bit code, where it allowed to avoid a conditional jump. Here we have to use a jump anyway). Signed-off-by: Denys Vlasenko <[email protected]> CC: Linus Torvalds <[email protected]> CC: Steven Rostedt <[email protected]> CC: Ingo Molnar <[email protected]> CC: Borislav Petkov <[email protected]> CC: "H. Peter Anvin" <[email protected]> CC: Andy Lutomirski <[email protected]> CC: Oleg Nesterov <[email protected]> CC: Frederic Weisbecker <[email protected]> CC: Alexei Starovoitov <[email protected]> CC: Will Drewry <[email protected]> CC: Kees Cook <[email protected]> CC: [email protected] CC: [email protected] --- arch/x86/entry/entry_64_compat.S | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index bb187a6..96f33a4 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -213,12 +213,13 @@ sysexit_from_sys_call: jnz ia32_ret_from_sys_call TRACE_IRQS_ON ENABLE_INTERRUPTS(CLBR_NONE) + xor %edi, %edi movl %eax, %esi /* second arg, syscall return value */ cmpl $-MAX_ERRNO, %eax /* is it an error ? */ jbe 1f movslq %eax, %rsi /* if error sign extend to 64 bits */ -1: setbe %al /* 1 if error, 0 if not */ - movzbl %al, %edi /* zero-extend that into %edi */ + inc %edi +1: /* edi: 1 if error, 0 if not */ call __audit_syscall_exit movq RAX(%rsp), %rax /* reload syscall return value */ movl $(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), %edi -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
