RE: [Xen-users] Which Permutations of 64/32 work?

[Geoff Streeter <geoff@xxxxxxxxxx>]

At 2006-09-25 11:30 +0200, Petersson, Mats wrote:
> > -----Original Message-----
> > From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> > [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Derek
> > Sent: 24 September 2006 19:02
> > To: xen ml
> > Subject: [Xen-users] Which Permutations of 64/32 work?
> >
> > Hi,
> >
> > I'm sure this must be documented somewhere, but I can't find it.
> >
> > Is there a document somewhere that shows which permutations of 64/32
> > bits are known to work or known not to work with XEN 3.0.2 on 64-bit
> > hardware?
> >
> > I.e. which of the following eight permutations work:
> >
> >   - 64-bit dom0 OS with 64 bit HVM domU OS    (?)
> Yes.
> >   - 64-bit dom0 OS with 32 bit HVM domU OS    (?)
> Yes.
> >   - 64-bit dom0 OS with 64 bit paravirt domU OS (?)
> Yes.
> >   - 64-bit dom0 OS with 32 bit paravirt domU OS (?)
> No.
> >   - 32-bit dom0 OS with 64 bit HVM domU OS    (?)
> No.
> >   - 32-bit dom0 OS with 32 bit HVM domU OS    (WORKS)
> Yes.
> >   - 32-bit dom0 OS with 64 bit paravirt domU OS  (?)
> No.
> >   - 32-bit dom0 OS with 32 bit paravirt domU OS  (WORKS)
> Yes.
>
> You've also missed the "intermediate" 32-bit PAE mode [supports > 4GB
> RAM for the Dom0 and DomU, but at any given time 4GB only is visible],
> which applies as:
> 32-bit PAE Dom0 + 32-bit PAE HVM DomU.
> Yes.
> 32-bit PAE Dom0 + 32-bit HVM DomU.
> Yes.
> 32-bit PAE Dom0 + 32-bit PAE PV DomU.
> Yes.
> All other combinations (32-bit PV DomU, 64-bit any DomU) doesn't work.
>
> The generic rule is:
> For Paravirtual, the guest has to be equal to Dom0.
> For HVM, the guest has to be equal or less than Dom0.
> Xen Hypervisor itself must be EQUAL to Dom0.
>
> To explain how this works: Xen itself must be able to access the same
> memory as the guest domain [Dom0 and DomU]. In the case of full (HVM)
> virtualization, it's possible to have different page-tables (and thus
> different "bitness") for the guest than the hypervisor, because the
> transition (VMRUN/VMEXIT) swaps CR3 (Page-table base address)[1] when
> switching between DomU and Xen. In Paravirtual, the same CR3 is used for
> both guest and hypervisor at the same time, which in turn means that
> they have to be in the same mode.
>
> This is of course assuming there are no bugs in the Xen hypervisor or
> tools that cause some particular combination(s) to be broken - but that
> applies to all things working/not working!
>
> [1] If you're really into how 32-bit/32-bit PAE/64-bit page-tables work,
> you'll be aware that not ONLY CR3 is affecting the operation of the
> page-tables, and as you may expect, the remaining registers that do
> affect this are ALSO swapped during the transition from/to guest.
>
> >
> > If this is not documented somewhere, maybe folks on this list wouldn't
> > mind replying with any knowledge they may have.  For example, if you
> > know one works because you've tried it yourself, or you know one
> > doesn't because it's excluded by some architectural design decision,
> > or whatever.
>
> I'm not sure if it's documented "properly" somewhere, but certainly it
> has been discussed previously.
>
> --
> Mats
My experience with Suse 10.1 64 bit Dom0 (I think) and Windows 64 bit HVM 
is that it doesn't work with Xen 3.0.2.
Anybody know better? 

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