Commit 262fc47a ('xen/balloon: don't use PV mode extra memory for zone
device allocations') removed the addition of the extra memory ranges to the
unpopulated range allocator, using those only for the balloon driver.

This forces the unpopulated allocator to attach hotplug ranges even when spare
memory (as part of the extra memory ranges) is available.  Furthermore, on PVH
domains it defeats the purpose of commit 38620fc4 ('x86/xen: attempt to
inflate the memory balloon on PVH'), as extra memory ranges would only be
used to map foreign memory if the kernel is built without XEN_UNPOPULATED_ALLOC
support.

Fix this by adding a helpers that adds the extra memory ranges to the list of
unpopulated pages, and zeroes the ranges so they are not also consumed by the
balloon driver.

This should have been part of 38620fc4, hence the fixes tag.

Note the current logic relies on unpopulated_init() (and hence
arch_xen_unpopulated_init()) always being called ahead of balloon_init(), so
that the extra memory regions are consumed by arch_xen_unpopulated_init().

Fixes: 38620fc4 ('x86/xen: attempt to inflate the memory balloon on PVH')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20240429155053.72509-1-roger.pau@citrix.comSigned-off-by: Juergen Gross <jgross@suse.com>