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Re: [Xen-devel] Copy-on-write memory to allow many more xenU domains per

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Subject: Re: [Xen-devel] Copy-on-write memory to allow many more xenU domains per machine
From: Michael Vrable <mvrable@xxxxxxxxxxx>
Date: Mon, 25 Oct 2004 15:38:10 -0700
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In-reply-to: <417CEDA6.6010209@xxxxxxxxxxxxxx>; from mpah@xxxxxxxxxxxxxx on Mon, Oct 25, 2004 at 01:12:22PM +0100
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On Mon, Oct 25, 2004 at 01:12:22PM +0100, Peri Hankey wrote:
> Some time ago Michael Vrable talked about copy-on-write memory to enable 
> large numbers of nearly identical machines to run on the same physical
> hardware. Is this a feasible proposition? I asked a few days ago in the 
> original thread, but no-one seems to have noticed. It seems to me that 
> it would be a very significant feature to offer.

I'm still around, though a bit slow to respond.

Ian mentioned that honeypots are one the scenarios where copy-on-write
sharing of memory makes a lot of sense; that's the reason that I'm
trying implement it.  I hope, of course, that if I do manage to get
something working it would be useful in other scenarios as well.

My current status: I've been working first on using shadow page tables
to hide the actual machine addresses of pages from domains.  I'm only
partway through doing that, and may yet go back and change how I'm
approaching that (I'm still learning bits about how Xen works as I'm
going through to modify the code; I'm also thinking now of changes that
would be good to make copy-on-write easier later).  I'm trying to write
up some of my design thoughts; if anyone here is interested or would
like to offer comments, I can post that here.

David mentioned implementing a clone() for VMs; that is something that
I'd also like to try out.  (It should be possible to test that after
only the first step of hiding machine addresses, before copy-on-write is
implemented, though copy-on-write will make it efficient.)

If someone else beats me to getting this implemented, I suppose I can't
complain.  But if this is an area that isn't getting much current work,
and I'm able to contribute (a hopefully clean implementation that others
wouldn't mind using), that would be great.

--Michael Vrable

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