Please see Michael Kerrisk on userspace ABI updates. A nice little manpage for this gimmick would be helpful, and maybe help other platforms that want one aswell to implement the same API.
On Mon, Apr 28, 2008 at 01:52:31PM +1000, Paul Mackerras wrote: > 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. > > Signed-off-by: Paul Mackerras <[EMAIL PROTECTED]> > --- > > diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S > index 215973a..2eb49a7 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) Am I missing something here or does this add a branch for every normal syscall? > 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) > > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@ozlabs.org > https://ozlabs.org/mailman/listinfo/linuxppc-dev ---end quoted text--- _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-dev
