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RE: [Xen-devel] Essay on an important Xen decision (long)

To: "Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>
Subject: RE: [Xen-devel] Essay on an important Xen decision (long)
From: Hollis Blanchard <hollisb@xxxxxxxxxx>
Date: Wed, 11 Jan 2006 15:36:03 -0600
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On Tue, 2006-01-10 at 16:39 -0800, Magenheimer, Dan (HP Labs Fort
Collins) wrote:
> > So ia64 dom0 physical 0 is machine 0? Where does Xen live in machine
> > space?
> > 
> > PowerPC exception handlers are architecturally hardcoded to the first
> > couple pages of memory, so Xen needs to live there. Linux 
> > expects it is
> > booting at 0 of course, so dom0 runs in an offset physical address
> > space.
> On ia64, Xen (and Linux when booting natively) is relocatable.
> Machine address 0 is not special on ia64 like it is on PowerPC.

Right, so P==M for dom0 (or any domain) will not work on PowerPC.
> Per the previous exchange with Anthony, there are many advantages
> to being able to move memory around invisibly to domains, which
> is easy with VP and much harder with P2M.  The current debate on
> Xen/ia64 is just for domain0 but it could expand...

As far as I can see, dom0 must be aware of the machine address space, so
that means P2M for PowerPC. dom0 is a special case: do you really need
to worry about migrating dom0, or memory compacting with other domains?

As for the question of domU being VP or P2M, I see no reason it
shouldn't be VP. IO-capable domUs (driver domains) could be VP with
proper IOMMU support. The PowerPC PAPR and Xen/ia64 implementations
demonstrate that this works...

Hollis Blanchard
IBM Linux Technology Center

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