linux-kernel - Re: Xen MMU's requirement to pin pages RO and initial_memory

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Message-ID: <20110517181716.GA4490@dumpdata.com>
Date:	Tue, 17 May 2011 14:17:16 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	"yinghai@...nel.org" <yinghai@...nel.org>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	Ian Campbell <Ian.Campbell@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: Xen MMU's requirement to pin pages RO and initial_memory_mapping.

> > xen_alloc_pte_init and xen_alloc_pmd_init are the ones that mark the
> > pagetable pages RO and pin them, calling make_lowmem_page_readonly and
> > pin_pagetable_pfn.
> > 
> > alloc_pte/pmd are called right before hooking them into the pagetable;
> > unfortunately that means that they fail at marking the pagetable pages
> > RO: make_lowmem_page_readonly uses lookup_address to find the pte
> > corresponding to a page, however at this point the pagetable pages are
> > not mapped yet (usually they are not hooked but when they are hooked, the
> > upper level pagetable page is not hooked), so lookup_address fails.
> 
> Right. We don't have to walk the hooked pagetable, I think. We are passed
> in the PMD/PGD of the PFN and we could look at the content of that PFN.

err, got that backwards. "passed in the PFN of the PMD/PGD".
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