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Subject: Re: [OMPI devel] Compiling OpenMPI 1.5.4 on Debian 6 qemu arm6l
From: Leif Lindholm (Leif.Lindholm_at_[hidden])
Date: 2012-02-28 07:50:57


Hi Ron,

Excellent work! Indeed - simply dropping the DMBs can lead to memory consistency issues even on ARMv6.

The architectural semantics for memory barriers exist in ARMv6 though - they just weren't given dedicated mnemonics.
What you could do is to simply replace the inline "dmb" sequences with inline cp15 operations:
- "MCR p15, 0, r0, c7, c10, 5"
  (the 'r0' is an encoding artefact and doesn't affect the register
  contents)

LDREXD/STREXD weren't part of the ARMv6 base architecture, although they are supported by the 1176 which is used in the Raspberry PI. If your tools support detecting/building for extension subarchitecture ARMv6k (supported by 1176), you can actually keep the 64-bit atomics in.

Best Regards,

Leif

References:
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0301h/Babfdddg.html
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0301h/Babhejba.html

> -----Original Message-----
> From: Jeffrey Squyres [mailto:jsquyres_at_[hidden]]
> Sent: 28 February 2012 12:30
> To: Ron Broberg; Open MPI Developers
> Cc: Leif Lindholm
> Subject: Re: [OMPI devel] Compiling OpenMPI 1.5.4 on Debian 6 qemu
> arm6l
>
> Ron -- Many thanks!
>
> Leif -- can you comment on this? (yes, I'm passing the buck to our ARM
> Open MPI representative :-) )
>
>
> On Feb 26, 2012, at 1:22 PM, Ron Broberg wrote:
>
> > I would like to report the following information regarding compiling
> OpenMPI on Debian ARMv6. I won't submit this as a patch because I don't
> believe that "delete all 'dmb' instructions" can be considered a well
> developed patch. But this information may be of use to someone down the
> line.
> >
> > I was able to compile the upstream openmpi-1.5.4 distribution on a
> Debian6 armv6l qemu emulation.
> > http://www.open-mpi.org/software/ompi/v1.4/downloads/openmpi-
> 1.4.5.tar.bz2
> >
> >
> > You have to make 3 changes to the package
> > 1) Delete all references to the RISC instruction 'dmb'
> > 2) Modify the 'configure' file to include an 'armv6' option
> > 3) Compile with CFLAGS=-march=armv6
> >
> > ## 1) make the following edits to these three files
> > ./opal/asm/generated/atomic-local.s
> > delete all dmb instructions
> > ./opal/asm/base/ARM.asm
> > delete all dmb instructions
> > ./opal/include/opal/sys/arm/atomic.h
> > change the lines:
> > #if OPAL_WANT_SMP_LOCKS
> > #define MB() __asm__ __volatile__ ("dmb" : : : "memory")
> > #define RMB() __asm__ __volatile__ ("dmb" : : : "memory")
> > #define WMB() __asm__ __volatile__ ("dmb" : : : "memory")
> > #else
> > #define MB()
> > #define RMB()
> > #define WMB()
> > #endif
> >
> > to read:
> > #define MB()
> > #define RMB()
> > #define WMB()
> >
> > ## 2) add the following to the 'configure' file at line 26946 of
> 171183
> > goto line 26946, there should be an 'alpha-' section above and an
> 'armv7' below
> > insert the following
> > armv6*)
> > ompi_cv_asm_arch="ARM"
> > OPAL_ASM_SUPPORT_64BIT=0
> > OMPI_GCC_INLINE_ASSIGN='"mov %0, #0" : "=&r"(ret)'
> > ;;
> >
> > ## 3) compile and install with the following CFLAGS
> > CFLAGS=-march=armv6
> > ./configure CFLAGS=-march=armv6
> > make
> > sudo make install
> >
> > more information about my build at
> > http://rhinohide.wordpress.com/2012/02/26/openmpi-on-raspberry-pi/
> >
> >
> > _______________________________________________
> > devel mailing list
> > devel_at_[hidden]
> > http://www.open-mpi.org/mailman/listinfo.cgi/devel
>
>
> --
> Jeff Squyres
> jsquyres_at_[hidden]
> For corporate legal information go to:
> http://www.cisco.com/web/about/doing_business/legal/cri/
>

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