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/LlvmCodeGen/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 cu Adrian -- "Is there not promise of rain?" Ling Tan asked suddenly out of the darkness. There had been need of rain for many days. "Only a promise," Lao Er said. Pearl S. Buck - Dragon Seed
