linux-kernel - Re: [PATCH 4/5] x86, mm: Add set_memory

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Date:	Mon, 08 Sep 2014 12:42:02 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Yigal Korman <yigal@...xistor.com>,
	Juergen Gross <jgross@...e.com>, Arnd Bergmann <arnd@...db.de>,
	Thomas Gleixner <tglx@...utronix.de>, akpm@...uxfoundation.org,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Stefan Bader <stefan.bader@...onical.com>
Subject: Re: [PATCH 4/5] x86, mm: Add set_memory_wt() for WT

On Mon, 2014-09-08 at 10:23 -0700, Andy Lutomirski wrote:
> On Sep 8, 2014 8:18 AM, "Toshi Kani" <toshi.kani@...com> wrote:
> >
> > On Sun, 2014-09-07 at 09:49 -0700, Andy Lutomirski wrote:
> > > On Sun, Sep 7, 2014 at 1:49 AM, Yigal Korman <yigal@...xistor.com> wrote:
> > > > I think that what confused Andy (or at least me) is the documentation in Documentation/x86/pat.txt
> > > > If it's possible, can you please update pat.txt as part of the patch?
> > >
> > > Indeed.  That file seems to indicate several times that the intended
> > > use of set_memory_xyz is for RAM.
> >
> >
> > Good point.  pat.txt is correct that the "intended" use of
> > set_memory_xyz() is for RAM since there is no other way to set non-WB
> > attribute for RAM.  For reserved memory, one should call ioremap_xyz()
> > to map with the xyz attribute directly.  From the functionality POV,
> > set_memory_xyz() works for reserved memory, but such usage is not
> > intended.
> >
> > Should I drop the patch 4/5 until we can track the use of WT for RAM?
> 
> Probably not.  I can imagine someone ioremapping a huge chunk of
> NV-DIMM and then wanting to change some of it to WT.  Unless I've
> missed something (which is rather likely), the cleanest way to do this
> is with set_memory_wt.

Yeah, that sounds possible.

> If that happens, someone should update pat.txt to indicate that it's allowed.

Since it is unlikely that someone will update pat.txt later, I will
update it to:

-------------------------------------------------------------------
API                    |    RAM   |  ACPI,...  |  Reserved/Holes  |

-----------------------|----------|------------|------------------|
set_memory_wt          |    *1    |    --      |       WT         |
 set_memory_wb         |          |            |                  |


*1: -EINVAL due to the current limitation in reserve_memtype().


Thanks,
-Toshi


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