The main reason of this change is that unpopulated-alloc
code cannot be used in its current form on Arm, but there
is a desire to reuse it to avoid wasting real RAM pages
for the grant/foreign mappings.

The problem is that system "iomem_resource" is used for
the address space allocation, but the really unallocated
space can't be figured out precisely by the domain on Arm
without hypervisor involvement. For example, not all device
I/O regions are known by the time domain starts creating
grant/foreign mappings. And following the advise from
"iomem_resource" we might end up reusing these regions by
a mistake. So, the hypervisor which maintains the P2M for
the domain is in the best position to provide unused regions
of guest physical address space which could be safely used
to create grant/foreign mappings.

Introduce new helper arch_xen_unpopulated_init() which purpose
is to create specific Xen resource based on the memory regions
provided by the hypervisor to be used as unused space for Xen
scratch pages. If arch doesn't define arch_xen_unpopulated_init()
the default "iomem_resource" will be used.

Update the arguments list of allocate_resource() in fill_list()
to always allocate a region from the hotpluggable range
(maximum possible addressable physical memory range for which
the linear mapping could be created). If arch doesn't define
arch_get_mappable_range() the default range (0,-1) will be used.

The behaviour on x86 won't be changed by current patch as both
arch_xen_unpopulated_init() and arch_get_mappable_range()
are not implemented for it.

Also fallback to allocate xenballooned pages (balloon out RAM
pages) if we do not have any suitable resource to work with
(target_resource is invalid) and as the result we won't be able
to provide unpopulated pages on a request.
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/1639080336-26573-5-git-send-email-olekstysh@gmail.comSigned-off-by: Juergen Gross <jgross@suse.com>