Re: [PATCH] trim memory not covered by WB MTRRs

[Jesse Barnes]

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.

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