Commit 745a14cc authored Apr 28, 2008 by Paul Mackerras

[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>

parent a01e035e

arch/powerpc/kernel/head_64.S

+11 −0

Original line number	Diff line number	Diff line
		@@ -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)