linux-kernel - Re: [Xen-devel] Re: [PATCH 04/10] xen/mmu: For 1-1 mapping, automatically set _PAGE

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Date:	Wed, 22 Dec 2010 16:27:24 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Beulich <JBeulich@...ell.com>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [Xen-devel] Re: [PATCH 04/10] xen/mmu: For 1-1 mapping,
 automatically set _PAGE_IOMAP.

On Wed, 2010-12-22 at 15:02 +0000, Konrad Rzeszutek Wilk wrote:
> On Tue, Dec 21, 2010 at 02:29:31PM -0800, Jeremy Fitzhardinge wrote:
> > On 12/21/2010 01:37 PM, Konrad Rzeszutek Wilk wrote:
> > > @@ -832,6 +832,9 @@ static pteval_t pte_pfn_to_mfn(pteval_t val)
> > >  		pteval_t flags = val & PTE_FLAGS_MASK;
> > >  		unsigned long mfn = pfn_to_mfn(pfn);
> > >  
> > > +		if (mfn == pfn)
> > > +			flags |= _PAGE_IOMAP;
> > 
> > Why?  Does it really make sense to set _PAGE_IOMAP if they just happen
> > to be the same value?
> 
> Yes. Without this, user applications, such as 'dmidecode' cannot get
> data.

I think Jeremy's point is that the "mfn == pfn" here has false
positives, since it is possible that a normal RAM page has identical mfn
and pfn by pure chance.

Ian.


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