Author: kib
Date: Thu Sep 15 09:53:04 2011
New Revision: 225575
URL: http://svn.freebsd.org/changeset/base/225575
Log:
Microoptimize the return path for the fast syscalls on amd64. Arrange
the code to have the fall-through path to follow the likely target.
Do not use intermediate register to reload user %rsp.
Proposed by: alc
Reviewed by: alc, jhb
Approved by: re (bz)
MFC after: 2 weeks
Modified:
head/sys/amd64/amd64/exception.S
Modified: head/sys/amd64/amd64/exception.S
==============================================================================
--- head/sys/amd64/amd64/exception.S Thu Sep 15 09:22:49 2011
(r225574)
+++ head/sys/amd64/amd64/exception.S Thu Sep 15 09:53:04 2011
(r225575)
@@ -393,14 +393,8 @@ IDTVEC(fast_syscall)
/* Check for and handle AST's on return to userland. */
movq PCPU(CURTHREAD),%rax
testl $TDF_ASTPENDING | TDF_NEEDRESCHED,TD_FLAGS(%rax)
- je 2f
- sti
- movq %rsp, %rdi
- call ast
- jmp 1b
-
- .align 16
-2: /* Restore preserved registers. */
+ jne 2f
+ /* Restore preserved registers. */
MEXITCOUNT
movq TF_RDI(%rsp),%rdi /* bonus; preserve arg 1 */
movq TF_RSI(%rsp),%rsi /* bonus: preserve arg 2 */
@@ -408,10 +402,16 @@ IDTVEC(fast_syscall)
movq TF_RAX(%rsp),%rax /* return value 1 */
movq TF_RFLAGS(%rsp),%r11 /* original %rflags */
movq TF_RIP(%rsp),%rcx /* original %rip */
- movq TF_RSP(%rsp),%r9 /* user stack pointer */
- movq %r9,%rsp /* original %rsp */
+ movq TF_RSP(%rsp),%rsp /* user stack pointer */
swapgs
sysretq
+
+2: /* AST scheduled. */
+ sti
+ movq %rsp,%rdi
+ call ast
+ jmp 1b
+
3: /* Requested full context restore, use doreti for that. */
MEXITCOUNT
jmp doreti
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