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Re: [Xen-devel] Memory Sharing

To: Lakshitha Harshan <harshan.dll@xxxxxxxxx>
Subject: Re: [Xen-devel] Memory Sharing
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Wed, 1 Jun 2011 10:46:58 +0100
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At 09:15 +0530 on 01 Jun (1306919734), Lakshitha Harshan wrote:
> This is regarding memory page sharing.
> I traced the code for page_sharing_nominate. But couldn't find the exact
> location
> which requires HAP. So if somebody could tell me the location it would be a
> great help for me.

Grrr, it doesn't look like there is a test. :(  I'll add one. 

> My other questions are,
> I don't understand the role HAP plays in page sharing?
> Why can't we use the shadow page tables with page sharing?

Briefly: because nobody did the work to make it happen.  The shadow
pagetable code would need to be updated to deal with the p2m code
telling it that a page was shared (possibly just reporting a pagefault
as fixed would do).  I think it would also need a check that an in-use
top-level pagetable could never be shared or paged out, and I'm sure
there would be a few more bugs here and there.

It's possible that once Olaf and Keir have finished with the code to
suspend a vcpus in mid-action, the shadow pagetable code will just
work[tm] with memory sharing. 



Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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