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[POWERPC] Add fast little-endian switch system call

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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>
This commit is contained in:
Paul Mackerras 2008-04-28 13:52:31 +10:00
parent a01e035ebb
commit 745a14cc26
1 changed files with 11 additions and 0 deletions
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11
arch/powerpc/kernel/head_64.S
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@ -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)