x86-64: Slightly shorten line system call entry and exit paths

GET_THREAD_INFO() involves a memory read immediately followed by
an "sub" on the value read, in turn (in several cases)
immediately followed by a use of the calculated value as the
base address of a memory access. This combination of
instructions has a non-negligible potential for stalls.

In the system call entry point code, however, the (fixed) offset
of the stack pointer from the end of the stack is generally
known, and hence we can instead avoid the memory load and
subtract, and instead do the memory reference using %rsp as the
base register. To do so in a legible fashion, introduce a
THREAD_INFO() macro which, provided a register (generally %rsp)
and the known offset from the end of the stack, produces a
suitable memory access operand.

The patch attempts to only touch the fast paths (no auditing and
alike), but manages to do so only in the 64-bit entry point
case; the compatibility mode entry points have so many
interdependencies between their various branch targets that it
was necessary to also adjust the slow paths to eliminate the
risk of having missed some register dependency during code
analysis.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/4ED4CD690200007800064075@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 3 insertions, 5 deletions

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 1581f1990187..75f72a50cf26 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -478,8 +478,7 @@ ENTRY(system_call_after_swapgs)
         movq  %rax,ORIG_RAX-ARGOFFSET(%rsp)
         movq  %rcx,RIP-ARGOFFSET(%rsp)
         CFI_REL_OFFSET rip,RIP-ARGOFFSET
-        GET_THREAD_INFO(%rcx)
-        testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%rcx)
+        testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
         jnz tracesys
 system_call_fastpath:
         cmpq $__NR_syscall_max,%rax
@@ -496,10 +495,9 @@ ret_from_sys_call:
         /* edi: flagmask */
 sysret_check:
         LOCKDEP_SYS_EXIT
-        GET_THREAD_INFO(%rcx)
         DISABLE_INTERRUPTS(CLBR_NONE)
         TRACE_IRQS_OFF
-        movl TI_flags(%rcx),%edx
+        movl TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET),%edx
         andl %edi,%edx
         jnz  sysret_careful
         CFI_REMEMBER_STATE
@@ -583,7 +581,7 @@ sysret_audit:
         /* Do syscall tracing */
 tracesys:
 #ifdef CONFIG_AUDITSYSCALL
-        testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),TI_flags(%rcx)
+        testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
         jz auditsys
 #endif
         SAVE_REST