Re: [patch 02/11] PAT x86: Map only usable memory in x86_64 identity map and kernel text

["Siddha, Suresh B" <suresh.b.siddha@intel.com>]

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