In order to properly monitor resource use, Xen maintains information on
the grant mappings a domain may create to map grants offered by other
domains.  In the process of carrying out certain actions, Xen would
iterate over all such entries, including ones which aren't in use
anymore and some which may have been created but never used.  If the
number of entries for a given domain is large enough, this iterating of
the entire table may tie up a CPU for too long, starving other domains
or causing issues in the hypervisor itself.