Charlie Jacobsen authored Feb 15, 2016 and Vikram Narayanan committed Oct 26, 2016

This is kind of like an mmap region, but only for RAM. (The guest
virtual page tables will be configured for write back).

Use case: You get a RAM capability from some other guy, and you
want to map it into your physical/virtual address space. This is
reasonable to figure out manually for big RAM chunks that are
mapped one/two times. But for little tedious ones (string sharing),
it's helpful to have an allocator to assist and track free parts
of guest physical.

The RAM map region uses very course-grained physical address space
allocation (minimum 64 MBs). This leads to a lot of internal
fragmentation, but it should be tolerable since the region is big.
It also means we have a smaller bit of metadata for tracking the
region.