As I think has been mentioned here a few times before, Xen is able to support guests with more RAM than the host's (strictly, dom0's) address space. For example, 64-bit guests with >4G RAM on 32-bit hosts. For this and for other reasons, guest physical RAM is not mapped into any host process.
I don't expect qemu to take the Xen mapcache, which has been extensively discussed and is apparently not well-regarded here. However, it would be very helpful if where reasonable parts of qemu would avoid assuming that they can get at guest physical memory by use of phys_ram_base. For example, in the loader in pc.c, simply adding phys_ram_base does not work and we have to have a rather large patch to convert things to use cpu_physical_memory_rw. The result is no more cumbersome - indeed, it's slightly tidier in a few ways because there's less need to constantly add and subtract phys_ram_base; the code can just deal with guest physical addresses directly, as numbers, and leave the actual memory access to the existing physical memory abstraction. So I think it would be nice to have that change in qemu upstream. While it doesn't directly enable anything useful right away, the result is slightly cleaner. I'll submit a proper patch shortly. Ian.