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authorPaul Mackerras <paulus@samba.org>2008-04-28 13:52:31 +1000
committerPaul Mackerras <paulus@samba.org>2008-04-29 15:57:34 +1000
commit745a14cc264b1832c638e41812e0cb04328b2db1 (patch)
tree427272e0a4a1f3df96f007b1df53b73d572ec63e /arch/powerpc/kernel/head_64.S
parenta01e035ebb552223c03f2d9138ffc73f2d4d3965 (diff)
[POWERPC] Add fast little-endian switch system call
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. The other thing about this system call is that it doesn't clobber as many registers as a normal system call. It only clobbers r12. Signed-off-by: Paul Mackerras <paulus@samba.org>
Diffstat (limited to 'arch/powerpc/kernel/head_64.S')
-rw-r--r--arch/powerpc/kernel/head_64.S11
1 files changed, 11 insertions, 0 deletions
diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 215973a2c8d..024805e1747 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)