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

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Message-ID: <1409139836.17054.1517936752110.JavaMail.zimbra@efficios.com>
Date:   Tue, 6 Feb 2018 17:05:52 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Will Deacon <will.deacon@....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 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 ?

Thanks,

Mathieu


> 
> Will

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com