Baffled by multipage_allocation_permitted()

[Dave McCracken <dcm@mccr.org>]

I have completed changes that allow me to create/save/restore domains 
allocated with 2M pages and run kernels with hugepage support.  This all 
appears to work fine.

However, while teaching the balloon driver to work in 2M page sizes, I 
suddenly had the multipage_allocation_permitted() macro in the hypervisor 
start failing.  This macro has the definition:

#define multipage_allocation_permitted(d)   \
   (!rangeset_is_empty((d)->iomem_caps) || \
    !rangeset_is_empty((d)->arch.ioport_caps))

I had never tracked down the significance of this macro since it didn't appear 
to be an issue.  Now it's suddenly denying me the ability to work in 2M pages.

What is this macro trying to achieve?  Why is it necessary?  Is there any 
compelling reason we can't just remove it and allow multipage allocations from 
any domain?

Thanks,
Dave McCracken
Oracle Corp.