RE: [Xen-devel] Question about implementation of 32-bit guests on64-bit hypervisor (IDT-related)

["Vessey, Bruce A" <Bruce.Vessey@xxxxxxxxxx>]

I said there would probably be more questions.  :-)

We have some confusion regarding the assignment of hardware physical
addresses to the HVMs.  If all 32-bit HVMs are run in legacy mode, then
they would be restricted to using 32-bit PAE shadow page tables.  The
maximum physical addressing of such tables is limited to 64GB, so this
implies that all 32-bit HVMs must be located within the first 64GB of
hardware physical memory.  However, your previous responses indicated
that 32-bit HVMs do not have such a restriction.  So our conclusion is
that 32-bit HVMs running in legacy mode using 32-bit PAE page tables are
able to address hardware physical memory above the 64GB limit.  Is this
correct?  Could we create two separate 32-bit HVM guests, assign each of
them 64GB of memory, and have them run concurrently without any problems
(assuming that the platform has sufficient memory installed)?

Thanks again,
Bruce


-----Original Message-----
From: Vessey, Bruce A 
Sent: Friday, December 07, 2007 1:27 PM
To: 'Mark Williamson'; xen-devel@xxxxxxxxxxxxxxxxxxx
Cc: Ian Pratt; Keir Fraser
Subject: RE: [Xen-devel] Question about implementation of 32-bit guests
on64-bit hypervisor (IDT-related)

Thanks to all of you for your answers.  I think we'll have some more
questions, but we need to figure out what they are first.  

          - Bruce



-----Original Message-----
From: M.A. Williamson [mailto:maw48@xxxxxxxxxxxxxxxx] On Behalf Of Mark
Williamson
Sent: Friday, December 07, 2007 11:52 AM
To: xen-devel@xxxxxxxxxxxxxxxxxxx
Cc: Ian Pratt; Keir Fraser; Vessey, Bruce A
Subject: Re: [Xen-devel] Question about implementation of 32-bit guests
on64-bit hypervisor (IDT-related)

> > > guests on top of the 64-bit hypervisor?
> >
> > 32-bit PV guests are restricted to using the low 128GB of physical 
> > memory.
> > There's not even that restriction for 32-bit HVM guests.
>
> We could even use shadow translate mode to get around the 128GB 
> restriction, though at some performance cost. I guess you'd switch the

> guest to shadow mode if you've run out of memory below 128GB.

That's not possible right now, though, right?  Shadowing PV guests was
still broken IIRC?

Cheers,
Mark


--
Dave: Just a question. What use is a unicyle with no seat?  And no
pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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