On Tue, Feb 06, 2018 at 05:05:52PM +0000, Mathieu Desnoyers wrote: > ----- On Feb 6, 2018, at 9:11 AM, Will Deacon will.dea...@arm.com wrote: > > > On Tue, Feb 06, 2018 at 02:06:50PM +0000, Mathieu Desnoyers wrote: > >> ----- On Feb 6, 2018, at 8:55 AM, Will Deacon will.dea...@arm.com wrote: > >> > On Tue, Feb 06, 2018 at 12:52:34PM +0000, Mathieu Desnoyers wrote: > >> >> One approach I would consider for this is to duplicate this > >> >> comment and add it just above the "eret" instruction within the > >> >> macro: > >> >> > >> >> /* > >> >> * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on eret context > >> >> synchronization > >> >> * when returning from IPI handler, and when returning to > >> >> user-space. > >> >> */ > >> >> > >> >> Or perhaps Will has something else in mind ? > >> > > >> > To be honest with you, I'd just drop the comment entirely. entry.S is > >> > terrifying these days and nobody should have to go in there to understand > >> > why we select ARCH_HAS_MEMBARRIER_SYNC_CORE. If you really feel a > >> > justification > >> > is needed, I'd be happy with a line in the Kconfig file. > >> > >> My concern is that someone wanting to optimize away a few cycles by > >> changing > >> eret to something else in the future will not be looking at Kconfig: that > >> person will be staring at entry.S. > > > > That person will probably also be me, or somebody who sits within punching > > distance. I really wouldn't worry about it. There a bunch of other > > things that will break if we don't use ERET here and, if it's a real > > concern, we're making the *huge* assumption that developers actually > > read and pay attention to comments. > > > >> One alternative presented by PeterZ on irc is to do like ppc: define a > >> macro for eret, and stick all appropriate comments near the macro. This > >> way, it won't hurt when reading the code, but at least it keeps the > >> comments near the instruction being discussed. > > > > For the sake of avoiding the conflict, can we just drop it for now, please? > > Having an "eret" macro isn't obvious, because people won't realise that it's > > a macro. Having "exception_return" is cryptic as hell to people familiar > > with the ISA. > > I'd be OK not adding comments in the assembly provided that we document this > within the new documentation file as I just posted as RFC: > > http://lkml.kernel.org/r/1517936413-19675-1-git-send-email-mathieu.desnoy...@efficios.com > > Thoughts ?
I certainly think that Documentation/ and probably init/Kconfig are the right places to describe this, but I defer to Ingo on whether or not arch-support.txt is ok with free-form text comments. Will
