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RE: [Xen-devel] PAE support revisited

To: "Karen White" <kawhite@xxxxxxxxxx>, "Nuutti Kotivuori" <naked@xxxxxx>
Subject: RE: [Xen-devel] PAE support revisited
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Tue, 22 Mar 2005 08:00:04 -0000
Cc: <xen-devel@xxxxxxxxxxxxxxxxxxxxx>, <ian.pratt@xxxxxxxxxxxx>
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Thread-topic: [Xen-devel] PAE support revisited
> There are at least two separate efforts looking at enabling PAE in
> Xen that I know of.  A couple of us are looking at it and we have
> started correspondance with Gerd Knorr, who is also looking at it.
> We have no firm idea on when a working implementation will
> be available, but are hoping sooner rather than later.

There are a few things that need to be sorted to get PAE to work:

 * cleanup use of 'unsigned long' that exists in some handling of PTEs.
We've agreed a strategy of doing this with Gerd
 * change hypercall structs (possibly just switch to using x86_64 mem op
hypercall layout)

 * Modify Linux arch xen to using a variable sized hypervisor hole in
the top of the VA space. Possibly best done by moving fixaddr to the
bottom end of memory.

 * adjust hypervisor memory layout to deal with larger variable sized
frame table.

 * add 3-level type checking to Xen (this should be easy as it already
supports 2 and 4 level)

 * add 3-level support to Linux arch Xen (this should be easy as the
there are very few modifications to the mm system these days) 

Once its working, there are a few other features that would be nice:

 * shadow mode support to run 2 level guests on a 3 level hypervisor

 * Linux kernel that boot time switches between 2 and 3 level. I'm sure
the distros would love this on native x86. I believe it can be done with
low performance cost by using function dispatch patching to switch
between 2 and 3 level versions of Linux's performance critical functions
(e.g. the _range functions and fork), and then just using 'if'
statements in the pte macros used by the less critical functions.  

It would be great if other people could start tackling some of the items
on the list.


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