"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 <dvlas...@redhat.com>
CC: Linus Torvalds <torva...@linux-foundation.org>
CC: Steven Rostedt <rost...@goodmis.org>
CC: Ingo Molnar <mi...@kernel.org>
CC: Borislav Petkov <b...@alien8.de>
CC: "H. Peter Anvin" <h...@zytor.com>
CC: Andy Lutomirski <l...@amacapital.net>
CC: Oleg Nesterov <o...@redhat.com>
CC: Frederic Weisbecker <fweis...@gmail.com>
CC: Alexei Starovoitov <a...@plumgrid.com>
CC: Will Drewry <w...@chromium.org>
CC: Kees Cook <keesc...@chromium.org>
CC: x...@kernel.org
CC: linux-kernel@vger.kernel.org
---
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
--
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