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Re: [Xen-devel] Readonly memory for guest domain

To: "Peter Teoh" <htmldeveloper@xxxxxxxxx>
Subject: Re: [Xen-devel] Readonly memory for guest domain
From: "pradeep singh rautela" <rautelap@xxxxxxxxx>
Date: Thu, 13 Sep 2007 10:10:12 +0530
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
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On 9/13/07, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote:
> Thank you for the answer.   In the first place, we will not know what is
> pagetable or non-pagetable memory.   For example, during dom0/domU
> initialisation, the guest OS will query the e820 bios mechanism for physical
> memory  availability, and the guest OS (paravirt or HVM) will then assign
> different parts of the physical memory for pagetable construction.   Then
> after all the pagetable is completely constructed, the CR3 is loaded, which
> started the hardware MMU operation.    So therefore, before the CR3 is
> loaded the entire physical memory is marked as readonly, and after the CR3
> is loaded, only those memory not involved in pagetable mapping are unmarked
> readonly?
> Does not seem right, as guest OS can change the CR3 anytime subsequently as
> well.

Any writes to CR3 'll be trapped to the Xen itself AFAIK. So, yes any
guest can change the CR3 anytime but there is always Xen to see what
it is writing in the CR3 .Anything beyond the memory assigned to
domain is illegal, xen knows the limits of the domains.

Please CMIIW somewhere.

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pradeep singh rautela

"question = ( to ) ? be : ! be;"
                -- Wm. Shakespeare

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