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RE: [Xen-devel] More page-table questions.

To: "Keir Fraser" <keir@xxxxxxxxxxxxx>, "PUCCETTI Armand" <armand.puccetti@xxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] More page-table questions.
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Mon, 12 Mar 2007 17:26:23 +0100
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Thread-topic: [Xen-devel] More page-table questions.

> -----Original Message-----
> From: Keir Fraser [mailto:keir@xxxxxxxxxxxxx] 
> Sent: 12 March 2007 16:23
> To: Petersson, Mats; PUCCETTI Armand; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Re: Xen-devel Digest, Vol 25, Issue 93
> On 12/3/07 16:19, "Petersson, Mats" <Mats.Petersson@xxxxxxx> wrote:
> >> I have seen in the code that there are different cases of 
> guest+host
> >> paging table heights. Why?
> > 
> > I'm sorry, I don't quite know this. I believe that the 
> page-table has to
> > be the same number of levels in both Xen and the PV guest.
> > 
> > There's been some recent work to implement 32-bit PV on 
> 64-bit HV, which
> > I think changes this by allowing a 32-bit PAE guest to run 
> on a 64-bit
> > hypervisor. Someone else who works more on PV is probably better to
> > answer this... 
> For PV guests, there are no separate Xen/shadow page tables. 
> Xen reserves a
> bit of space at the top end of guest pagetables to map itself. Hence
> normally the guest and Xen pagetables must be the same height 
> as they are
> actually the same pagetables.
> Supporting PAE guest on 64-bit Xen is the only exception. Xen 
> maintains a
> hidden top-level page directory and one of the entries in 
> that directory
> points at the guest's three-level pagetable. But again there 
> is no shadowing
> of the guest three-level pagetable: they are directly hooked 
> into the hidden
> top-level directory, and the real physical %cr3 points at that hidden
> directory.

Are the page-tables ever updated directly by the guest, or is it all
done via hyper-calls?


>  -- Keir

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