WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

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
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Delivery-date: Tue, 21 Apr 2009 01:37:49 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <67822.80752.qm@xxxxxxxxxxxxxxxxxxxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <67822.80752.qm@xxxxxxxxxxxxxxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.17 (2007-11-01)
Hi, 

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
time. 

Cheers,

Tim.

> Thanks.
> Priya
> 
> 
>       
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel

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

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel