linux-kernel - Re: [PATCH v2 16/18] x86: io: implement dummy relaxed accessor macros for writes

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Message-ID: <20140523153409.GI21319@arm.com>
Date:	Fri, 23 May 2014 16:34:09 +0100
From:	Will Deacon <will.deacon@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"monstr@...str.eu" <monstr@...str.eu>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"broonie@...aro.org" <broonie@...aro.org>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 16/18] x86: io: implement dummy relaxed accessor
 macros for writes

On Fri, May 23, 2014 at 04:20:08PM +0100, H. Peter Anvin wrote:
> On 05/23/2014 07:57 AM, Will Deacon wrote:
> > On Fri, May 23, 2014 at 03:53:20PM +0100, H. Peter Anvin wrote:
> >> On 05/23/2014 07:46 AM, Will Deacon wrote:
> >>> I would like the relaxed accessors to be ordered with respect to each other...
> >>>
> >>> What do you think?
> >>>
> >>
> >> I think "I would like" isn't a very good motivation.  What are the
> >> semantics of these things supposed to be?  It seems more than a bit odd
> >> to require them to be ordered with respect to each other and everything
> >> else (which is what a memory clobber does) and then call them "relaxed".
> > 
> > I suggested some informal semantics in the cover letter:
> > 
> >   https://lkml.org/lkml/2014/4/17/269
> > 
> > Basically, if we define relaxed accesses not to be ordered against anything
> > apart from other accesses (relaxed or otherwise) to the same device, then
> > they become a tonne cheaper on arm/arm64/powerpc. Currently we have to
> > include expensive memory barriers in order to synchronise with accesses to
> > DMA buffers which is rarely needed.
> > 
> > For those requirements, I don't think we need the "memory" clobber for x86,
> > but would appreciate your views on this.
> > 
> 
> OK... first of all you didn't send the cover letter to the union of all
> the people you sent patches to, but second, documenting semantics in the
> one piece of the patchset that wouldn't make it into git is just about
> the worst possible place to put it.
> 
> This documentation is absolutely critical if we expect people to be able
> to use these correctly, including when additional barriers may be required.

There is also a documentation patch [1] in this series but, again, I didn't
CC everybody on it. Sorry, but the level of interest this sort of stuff
generates amongst kernel developers is close to zero so I only included
people I thought cared on CC for the entire series. I'm stuck between a rock
and a hard place trying to CC interested people whilst at the same time
trying to avoid spamming all the arch maintainers.

I'll add you to CC if/when I post a third version. In the meantime, it's
all archived on lkml and linux-arch.

> As far as x86 is concerned, in gcc volatiles are ordered with respect to
> each other, so as you say I don't think we need a memory clobber here.

Thanks for the confirmation, I'll put that patch back like it was
originally.

Will

[1] https://lkml.org/lkml/2014/5/22/464
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