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SubjectRe: [patch 02/11] PAT x86: Map only usable memory in x86_64 identity map and kernel text
On Mon, Jan 14, 2008 at 05:43:24PM +0100, Ingo Molnar wrote:
>
> * Pallipadi, Venkatesh <venkatesh.pallipadi@intel.com> wrote:
>
> > Also, relying on MTRR, is like giving more importance to BIOS writer
> > than required :-). I think the best way to deal with MTRR is just to
> > not touch it. Leave it as it is and do not try to assume that they are
> > correct, as frequently they will not be.
>
> i'd suggest the following strategy on PAT-capable CPUs:
>
> - do not try to write MTRRs. Ever.
>
> - _read_ the current MTRR settings (including the default MTRR) and
> check them against the e820 map. I can see two basic types of
> mismatches:
>
> - RAM area marked fine in e820 but marked UC by MTRR: this
> currently results in a slow system.

Time to resurrect Jesse's old patches
i386-trim-memory-not-covered-by-wb-mtrrs.patch(which was in -mm sometime back)

> (NOTE: UC- would be fine and
> overridable by PAT, hence it's not a conflict we should detect.)

UC- can't be specified by MTRR's.

> - mmio area marked cacheable in the MTRR (results in broken system)

PAT can help specify the UC/WC attribute here.

thanks,
suresh


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