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RE: [Xen-devel] 32/64-bit hypercall interface

Jeremy Katz wrote:
> On Fri, 2005-09-30 at 17:42 +0100, Keir Fraser wrote:
>> There's the rub: we don't expect to ever want to provide 32-bit x86
>> ABI compatibility on 64-bit x86 Xen. We will not be supporting 32-bit
>> paravirtualised guests on 64-bit x86 Xen,
> ... which I've previously said and continue to think is going to be
> something that will eventually be regretted.  People are going to want
> to continue to run in a 32bit OS for legacy reasons for quite a while
> (even with the compatibility you get on x86_64).  Making it so they
> can't do mix and match of 32 and 64 bit guests on a single host is
> going to significantly reduce the utility of Xen.
> Jeremy
Jeremy, Hi

That's mostly because of the H/W issues. One thing we could do there is
to run such  paravirtualised 32-bit guests in compatibility mode. But
they would need to use 4-level page tables, and there are other minor
differences. So those 32-bit guests wouldn't be really same as the ones
running on 32-bit Xen. I'm not sure that's what you wanted, but it's

I think a better way is to utilize H/W-based virtualization such as
Intel Virtualization Technology (VT) or AMD's Pacifica. That way we
should be able to use the same binaries (thus ABI) for both 32-bit
non-PAE and PAE domU on 64-bit x86 Xen. With H/W-based virtualization we
can run those 32-bit guests cleanly in 32-bit on 64-bit Xen. 32-bit
hypercalls from such 32-bit guests should be intercepted by a 32-bit
ring0 component that converts "int 0x82" based ones to VMCALL or VMMCALL
sent to 64-bit Xen (because such software interrupts won't cause
VMEXIT). There are other issues like impacts to the memory management in
Xen, but I think those can be handled as special cases of shadow mode
(i.e. don't make shadow pages). I don't think it would increase the
utility of Xen right away (because this requires new processors), but it
might be a sensible thing to do in the near future.

For full-virutalization, today we can mix 32 and 64 bit guests on x86_64
Xen (as in the xen-unstable tree) when the H/W-based virtualization is
present on the machine.

Intel Open Source Technology Center

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