linux-kernel - Re: [PATCH v2 6/6] x86, pat: Update documentation for WT changes

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Date:	Mon, 15 Sep 2014 15:19:11 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Juergen Gross <jgross@...e.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Yigal Korman <yigal@...xistor.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v2 6/6] x86, pat: Update documentation for WT changes

On Wed, 2014-09-10 at 15:34 -0600, Toshi Kani wrote:
> On Wed, 2014-09-10 at 13:29 -0700, Andy Lutomirski wrote:
> > On Wed, Sep 10, 2014 at 1:12 PM, Toshi Kani <toshi.kani@...com> wrote:
> > > On Wed, 2014-09-10 at 11:30 -0700, Andy Lutomirski wrote:
> > >> On Wed, Sep 10, 2014 at 9:51 AM, Toshi Kani <toshi.kani@...com> wrote:
> > >> > +Drivers may map the entire NV-DIMM range with ioremap_cache and then change
> > >> > +a specific range to wt with set_memory_wt.
> > >>
> > >> That's mighty specific :)
> > >
> > > How about below?
> > >
> > > Drivers may use set_memory_wt to set WT type for cached reserve ranges.
> > 
> > Do they have to be cached?
> 
> Yes, set_memory_xyz only supports WB->type->WB transition.
> 
> > How about:
> > 
> > Drivers may call set_memory_wt on ioremapped ranges.  In this case,
> > there is no need to change the memory type back before calling
> > iounmap.
> > 
> > (Or only on cached ioremapped ranges if that is, in fact, the case.)
> 
> Sounds good.  Yes, I will use cashed ioremapped ranges.

Well, testing "no need to change the memory type back before calling
iounmap" turns out to be a good test case.  I realized that
set_memory_xyz only works properly for RAM.  There are two problems for
using this interface for ioremapped ranges.

1) set_memory_xyz calls reserve_memtype() with __pa(addr).  However,
__pa() translates the addr into a fake physical address when it is an
ioremapped address.

2) reserve_memtype() does not work for set_memory_xyz.  For RAM, the WB
state is managed untracked.  Hence, WB->new->WB is not considered as a
conflict.  For ioremapped ranges, WB is tracked in the same way as other
cache types.  Hence, WB->new is considered as a conflict.

In my previous testing, 2) was undetected since 1) led using a fake
physical address which was not tracked for WB.  This made ioremapped
ranges worked just like RAM. :-( 

Anyway, 1) can be fixed by using slow_virt_to_phys() instead of __pa().
set_memory_xyz is already slow, but this makes it even slower, though.

For 2), WB has to be continuously tracked in order to detect aliasing,
ex. ioremap_cache and ioremap to a same address.  So, I think
reserve_memtype() needs the following changes:
 - Add a new arg to see if an operation is to create a new mapping or to
change cache attribute.
 - Track overlapping maps so that cache type change to an overlapping
range can be detected and failed.

This level of changes requires a separate set of patches if we pursue to
support ioremapped ranges.  So, I am considering to take one of the two
options below.

A) Drop the patch for set_memory_wt.

B) Keep the patch for set_memory_wt, but document that it fails with
-EINVAL and its use is for RAM only.

Any suggestion?
Thanks,
-Toshi

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