ARM: Avoid writing to control register on every exception

If we are not changing the control register value, avoid writing to it.
Writes to the control register can be very expensive, taking around a
hundred cycles or so.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 36276cdccfbc71e9e69de18fe21a00ac25ddbd07..3fe61de63e3014f87634c3d05717feae87bf8551 100644 (file)
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -321,6 +321,9 @@ ENDPROC(__pabt_svc)
  ARM(  stmib   sp, {r1 - r12}  )
  THUMB(        stmia   sp, {r0 - r12}  )
 
+ ATRAP(        mrc     p15, 0, r7, c1, c0, 0)
+ ATRAP(        ldr     r8, .LCcralign)
+
        ldmia   r0, {r3 - r5}
        add     r0, sp, #S_PC           @ here for interlock avoidance
        mov     r6, #-1                 @  ""  ""     ""        ""
@@ -328,6 +331,8 @@ ENDPROC(__pabt_svc)
        str     r3, [sp]                @ save the "real" r0 copied
                                        @ from the exception stack
 
+ ATRAP(        ldr     r8, [r8, #0])
+
        @
        @ We are now ready to fill in the remaining blanks on the stack:
        @
@@ -341,10 +346,9 @@ ENDPROC(__pabt_svc)
  ARM(  stmdb   r0, {sp, lr}^                   )
  THUMB(        store_user_sp_lr r0, r1, S_SP - S_PC    )
 
-       @
        @ Enable the alignment trap while in kernel mode
-       @
-       alignment_trap r0, .LCcralign
+ ATRAP(        teq     r8, r7)
+ ATRAP( mcrne  p15, 0, r8, c1, c0, 0)
 
        @
        @ Clear FP to mark the first stack frame

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index e52fe5a2d8439e36dc2e75bc3f5eb1a6df5ec752..6bb09d4abdea6356a575de475e0809765c3ee19e 100644 (file)
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -366,7 +366,7 @@ ENTRY(vector_swi)
        str     r0, [sp, #S_OLD_R0]             @ Save OLD_R0
 #endif
        zero_fp
-       alignment_trap ip, __cr_alignment
+       alignment_trap r10, ip, __cr_alignment
        enable_irq
        ct_user_exit
        get_thread_info tsk

diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index 8db307d0954bb03ee95550471dcc0e6f86ae560f..7b729394e78a30632f059f72aceb82a2660bfe82 100644 (file)
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S
@@ -37,11 +37,19 @@
 #endif
        .endm
 
-       .macro  alignment_trap, rtemp, label
 #ifdef CONFIG_ALIGNMENT_TRAP
-       ldr     \rtemp, \label
-       ldr     \rtemp, [\rtemp]
-       mcr     p15, 0, \rtemp, c1, c0
+#define ATRAP(x...) x
+#else
+#define ATRAP(x...)
+#endif
+
+       .macro  alignment_trap, rtmp1, rtmp2, label
+#ifdef CONFIG_ALIGNMENT_TRAP
+       mrc     p15, 0, \rtmp2, c1, c0, 0
+       ldr     \rtmp1, \label
+       ldr     \rtmp1, [\rtmp1]
+       teq     \rtmp1, \rtmp2
+       mcrne   p15, 0, \rtmp1, c1, c0, 0
 #endif
        .endm