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Re: [Xen-devel] Shadow Page Tables in Xen

To: priya sehgal <priyagps@xxxxxxxxxxx>
Subject: Re: [Xen-devel] Shadow Page Tables in Xen
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Tue, 21 Apr 2009 09:37:23 +0100
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At 20:34 +0100 on 20 Apr (1240259658), priya sehgal wrote:
> I was going through the memory management in Xen and little confused
> about shadow page tables for HVM guest and how they work.

Why are you interested?  Do you have a project in mind? 

> Few of my questions are :
> 1. A shadow page table, is a copy of the guest page table, with actual 
> machine frame numbers (MFN), as against the physical page number(PFN) in case 
> of guests. 
> Do the shadow page tables write protect each of the PTE it contains, or only 
> the pages containing the "guest page table" are write protected (to check if 
> the guest has modified any entry)? 

They write-protect all pagetables and some other things but not all of
memory; that wouldn't be very useful.

> 2. I know that live migration is supported on xen, but not sure if it works 
> for HVM guests. In this case also, xen will need to keep a shadow page table, 
> which should detect which pages have been modified since the last time the 
> pages were migrated. For this, shadow page table should mark all the PTEs as 
> write protected. Does xen implement live migration of HVM guests in this way 
> ? 
> If yes, then is this shadow table same as the one used in (1), or there are 
> multiple copies of the shadow tables for different purposes?

There's only one set of shadows, since the MMU can only use one at a



> Thanks.
> Priya
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Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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