From: Brad Peters <bpeters@redhat.com>
Date: Thu, 24 Jul 2008 18:05:55 -0400
Subject: [ppc] fast little endian implementation for System p AVE
Message-id: 20080724220555.17645.38170.sendpatchset@squad5-lp1.lab.bos.redhat.com
O-Subject: [PATCH RHEL5.3] Fast little endian implementation for System p AVE
Bugzilla: 439505
RH-Acked-by: David Howells <dhowells@redhat.com>

RHBZ#:
======
https://bugzilla.redhat.com/show_bug.cgi?id=439505

Description:
===========
New Feature / Power Arch Only

The feature improves the performance of Lx 86 which allows x86 binaries binaries to run on Power systems

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.

kABI Status:
============
No symbols were harmed.

Brew:
=====
Built on all platforms.
http://brewweb.devel.redhat.com/brew/taskinfo?taskID=1370977

Kernel binary rpm available at:
===============================
http://people.redhat.com/bpeters/kernels/kernel-2.6.18-94.el5.94.el5.439505.ppc64.rpm

Upstream Status:
================
Upstream at:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=745a14cc264b1832c638e41812e0cb04328b2db1

Test Status:
============
Tested using simple program which flips to little endian mode and back 1,000,000 times

  7/24/08 <bpeters@redhat.com>

===============================================================

Brad Peters 1-978-392-1000 x 23183
IBM on-site partner.

Proposed Patch:
===============
This patch is based on 2.6.18-94.el5

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 9f9e2c4..0b0db94 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -543,6 +543,10 @@ 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
@@ -557,6 +561,13 @@ 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)