This adds a system call on 64-bit platforms for switching between
little-endian and big-endian modes that is much faster than doing a
prctl call. This system call is handled as a special case right at
the start of the system call entry code, and because it is a special
case, it uses a system call number which is out of the range of
normal system calls, namely 0x1ebe.
Measurements with lmbench on a 4.2GHz POWER6 showed no measurable
change in the speed of normal system calls with this patch.
Switching endianness with this new system call takes around 60ns on a
4.2GHz POWER6, compared with around 300ns to switch endian mode with a
prctl. This can provide a significant performance advantage for
emulators for little-endian architectures that want to switch between
big-endian and little-endian mode frequently, e.g. because they are
generating instructions sequences on the fly and they want to run
those sequences in little-endian mode.
Signed-off-by: Paul Mackerras <[EMAIL PROTECTED]>
---
diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 215973a..2eb49a7 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -239,6 +239,10 @@ instruction_access_slb_pSeries:
.globl system_call_pSeries
system_call_pSeries:
HMT_MEDIUM
+BEGIN_FTR_SECTION
+ cmpdi r0,0x1ebe
+ beq- 1f
+END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE)
mr r9,r13
mfmsr r10
mfspr r13,SPRN_SPRG3
@@ -253,6 +257,13 @@ system_call_pSeries:
rfid
b . /* prevent speculative execution */
+/* Fast LE/BE switch system call */
+1: mfspr r12,SPRN_SRR1
+ xori r12,r12,MSR_LE
+ mtspr SPRN_SRR1,r12
+ rfid /* return to userspace */
+ b .
+
STD_EXCEPTION_PSERIES(0xd00, single_step)
STD_EXCEPTION_PSERIES(0xe00, trap_0e)
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