From 745a14cc264b1832c638e41812e0cb04328b2db1 Mon Sep 17 00:00:00 2001 From: Paul Mackerras Date: Mon, 28 Apr 2008 13:52:31 +1000 Subject: [PATCH] [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 --- arch/powerpc/kernel/head_64.S | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S index 215973a2c8..024805e174 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) -- 2.39.5