Re: CP15 Barrier emulation performance?
On Friday 28 July 2017 07:54:43 Adrian Bunk wrote:
> On Fri, Jul 28, 2017 at 08:30:30AM +0000, Riku Voipio wrote:
> > On Tue, Jul 11, 2017 at 09:45:49AM +0100, Ian Campbell wrote:
> > > On Tue, 2017-07-11 at 08:56 +0100, Edmund Grimley Evans wrote:
> > > > > Does this emulation take a considerable performance hit, as
> > > > > opposed to
> > > > > running on armhf hardware/kernel, where the instruction
> > > > > doesn't appear
> > > > > to be listed as deprecated?
> > > >
> > > > I'd expect the kernel-emulated instruction to be much slower
> > > > than any non-emulated instruction, but the overall effect on
> > > > performance will, of course, depend on how often the instruction
> > > > is used.
> > > >
> > > > See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=864847
> > > > (where I think I claimed that the instruction is deprecated on
> > > > armhf hardware but I could easily be wrong)
> > >
> > > Deprecated for both ARMv7 and ARMv8 according to the Kconfig help:
> > > config CP15_BARRIER_EMULATION
> > > bool "Emulate CP15 Barrier instructions"
> > > help
> > > The CP15 barrier instructions - CP15ISB, CP15DSB, and
> > > CP15DMB - are deprecated in ARMv8 (and ARMv7). It is
> > > strongly recommended to use the ISB, DSB, and DMB
> > > instructions instead.
> > >
> > > Say Y here to enable software emulation of these
> > > instructions for AArch32 userspace code. When this
> > > option is enabled, CP15 barrier usage is traced which can help
> > > identify software that needs updating.
> > >
> > > If unsure, say Y
> > >
> > > However I there is a sysctl which allows selecting between
> > > "undef", "emulated" and "hw" (where the latter is dependent on the
> > > hw actually supporting the instructions in question). See:
> > > http://elixir.free-electrons.com/linux/latest/source/Documentation
> > >/arm64/legacy_instructions.txt
> >
> > The ghc sources don't appeart to have direct CP15 instructions, so I
> > suspect ghc might be calling llvm with options that make it emit
> > CP15 barriers instead of DMB. Anyways look like a configuration
> > error rather than a coding error.
>
> https://sources.debian.net/src/ghc/8.0.2-5/compiler/llvmGen/LlvmCodeGe
>n/Ppr.hs/#L55
>
> ghc contains a hack to change that to arm-unknown-linux-gnueabi for
> armel.
>
> Changing it to armv7-unknown-linux-gnueabihf will likely fix this
> issue for armhf.
>
> > Riku
How can one go about discovering if the code we are running suffers from
this "bug"?
> cu
> Adrian
Thank you.
Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>
Reply to: