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. Riku
