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>
