From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from verein.lst.de (verein.lst.de [213.95.11.210]) (using TLSv1 with cipher EDH-RSA-DES-CBC3-SHA (168/168 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id DF5BBDDF28 for ; Tue, 29 Apr 2008 00:43:43 +1000 (EST) Date: Mon, 28 Apr 2008 16:43:32 +0200 From: Christoph Hellwig To: Paul Mackerras Subject: Re: [PATCH] Add fast little-endian switch system call Message-ID: <20080428144332.GA17109@lst.de> References: <18453.18943.906693.409245@cargo.ozlabs.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <18453.18943.906693.409245@cargo.ozlabs.ibm.com> Cc: linux-arch@vger.kernel.org, linuxppc-dev@ozlabs.org, mtk.manpages@gmail.com List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 > --- > > 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---