linux-kernel - Re: [patch 02/11] PAT x86: Map only usable memory in x86

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Date:	Tue, 15 Jan 2008 23:17:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Cc:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Andi Kleen <andi@...stfloor.org>, ebiederm@...ssion.com,
	rdreier@...co.com, torvalds@...ux-foundation.org, gregkh@...e.de,
	airlied@...net.ie, davej@...hat.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	jesse.barnes@...el.com
Subject: Re: [patch 02/11] PAT x86: Map only usable memory in x86_64
	identity map and kernel text


* Siddha, Suresh B <suresh.b.siddha@...el.com> wrote:

> On Mon, Jan 14, 2008 at 05:43:24PM +0100, Ingo Molnar wrote:
> > 
> > * Pallipadi, Venkatesh <venkatesh.pallipadi@...el.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)

just to make sure i understood the attribute priorities right: we cannot 
just mark it WB in the PAT and expect it to be write-back - the UC of 
the MTRR will control?

> >        (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.

hm, only by PATs? Not even by the default MTRR?

> >      - mmio area marked cacheable in the MTRR (results in broken 
> >      system)
> 
> PAT can help specify the UC/WC attribute here.

ok. So it seems we dont even need all that many special cases, a "dont 
write MTRRs" and "use PATs everywhere" rule would just do the right 
thing all across?

	Ingo
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