linux-kernel - Re: linux-next: manual merge of the tip tree with Linus' tree

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Message-ID: <20180206172156.GA25540@arm.com>
Date:   Tue, 6 Feb 2018 17:21:56 +0000
From:   Will Deacon <will.deacon@....com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: manual merge of the tip tree with Linus' tree

On Tue, Feb 06, 2018 at 05:05:52PM +0000, Mathieu Desnoyers wrote:
> ----- On Feb 6, 2018, at 9:11 AM, Will Deacon will.deacon@....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.deacon@....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.desnoyers@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