linux-kernel - Re: [patch 0/4] x86: PAT followup - Incremental changes and bug fixes

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Date:	Thu, 17 Jan 2008 23:52:43 +0100
From:	"Andreas Herrmann3" <andreas.herrmann3@....com>
To:	"Ingo Molnar" <mingo@...e.hu>
cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Venki Pallipadi" <venkatesh.pallipadi@...el.com>, ak@....de,
	ebiederm@...ssion.com, rdreier@...co.com,
	torvalds@...ux-foundation.org, gregkh@...e.de, airlied@...net.ie,
	davej@...hat.com, tglx@...utronix.de, hpa@...or.com,
	akpm@...ux-foundation.org, arjan@...radead.org,
	jesse.barnes@...el.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 0/4] x86: PAT followup - Incremental changes and bug
 fixes

On Thu, Jan 17, 2008 at 11:15:05PM +0100, Ingo Molnar wrote:
> 
> * Andreas Herrmann3 <andreas.herrmann3@....COM> wrote:
> 
> > On Thu, Jan 17, 2008 at 10:42:09PM +0100, Ingo Molnar wrote:
> > > 
> > > * Siddha, Suresh B <suresh.b.siddha@...el.com> wrote:
> > > 
> > > > On Thu, Jan 17, 2008 at 10:13:08PM +0100, Ingo Molnar wrote:
> > > > > but in general we must be robust enough in this case and just degrade 
> > > > > any overlapping page to UC (and emit a warning perhaps) - instead of 
> > > > > failing the ioremap and thus failing the driver (and the bootup).
> > > > 
> > > > But then, this will cause an attribute conflicit. Old one was 
> > > > specifying WB in PAT (ioremap with noflags) and the new ioremap 
> > > > specifies UC.
> > > 
> > > we could fix up all aliases of that page as well and degrade them to UC?
> > 
> > Yes, we must fix all aliases or reject the conflicting mapping. But 
> > fixing all aliases might not be that easy. (I've just seen a panic 
> > when using your patch ;-(
> 
> yes, indeed my patch is bad if you have PAT enabled: conflicting cache 
> attributes might be present. I'll go with your patch for now.

I think the best is to just reject conflicting mappings. (Because now
I am too tired to think about a safe way how to change the aliases to the
most restrictive memory type. ;-)

But then of course such boot-time problems like I've seen on my test
machines should be avoided somehow.


Andreas



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