From: Jesse Barnes <jbarnes@virtuousgeek.org>
To: Andi Kleen <andi@firstfloor.org>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs
Date: Tue, 26 Jun 2007 08:18:33 -0700
User-Agent: KMail/1.9.7
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
       Jesse Barnes <jesse.barnes@intel.com>, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org, Justin Piszcz <jpiszcz@lucidpixels.com>,
       Yinghai Lu <yhlu.kernel@gmail.com>
References: <200706251434.43863.jesse.barnes@intel.com> <20070626150348.GB7877@one.firstfloor.org> <200706260807.45362.jbarnes@virtuousgeek.org>
In-Reply-To: <200706260807.45362.jbarnes@virtuousgeek.org>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Message-Id: <200706260818.33759.jbarnes@virtuousgeek.org>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2081
Lines: 44

On Tuesday, June 26, 2007 8:07:45 am Jesse Barnes wrote:
> On Tuesday, June 26, 2007 8:03:48 am Andi Kleen wrote:
> > On Mon, Jun 25, 2007 at 08:30:52PM -0700, Jesse Barnes wrote:
> > > Oh, and FYI I've seen new systems with a default mapping type of WB,
> > > with a few uncached holes for MMIO.  That means PAT will be absolutely
> > > required on those systems if we want to use WC for the framebuffer or
> > > other memory.
> >
> > Why? As long as there are still MTRRs free you could use them for
> > WC even in this setup.
>
> According to the manual, you can't use WC or WT MTRRs on top of a WB MTRR
> range.  Only UC MTRRs can override WB MTRRs, but PAT doesn't have this
> restriction.

Actually, it looks like WT on top of WB is ok, but not WC on top of WB, 
quoting from the manual:

> 1. If the physical address falls within the first 1 MByte of physical
> memory and fixed MTRRs are enabled, the processor uses the memory type
> stored for the appropriate fixed-range MTRR.
>
> 2. Otherwise, the processor attempts to match the physical address with a
> memory type set by the variable-range MTRRs:
>
>     a. If one variable memory range matches, the processor uses the memory
>        type stored in the IA32_MTRR_PHYSBASEn register for that range.
>     b. If two or more variable memory ranges match and the memory types are
>        identical, then that memory type is used.
>     c. If two or more variable memory ranges match and one of the memory
>        types is UC, the UC memory type used.
>     d. If two or more variable memory ranges match and the memory types are
>        WT and WB, the WT memory type is used.
>     e. For overlaps not defined by the above rules, processor behavior is
>        undefined.
>
> 3. If no fixed or variable memory range matches, the processor uses the
> default memory type.

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/