query the page type of a hvm page from within a hvm guest

[Olaf Hering <olaf@aepfle.de>]

Now that kdump works with pv-on-hvm guests, one issue remains:
The balloon driver in the guest frees guest pages and marks them as
mmio. When the kernel crashes and the crash kernel attempts to read the
oldmem via /proc/vmcore a read from ballooned pages will generate 100%
load in dom0 because Xen asks qemu-dm for the page content. Since the
reads come in as 8byte requests each ballooned page is tried 512 times.

If the crash kernel had a way to ask the hypervisor wether a specific
guest gfn is ballooned and thus backed by ram, the load issue would not
happen.  There seems to be no interface to query the type of a guest gfn
from within the hvm guest.

Any ideas how to implement that?
I see HVMOP_set_mem_type, but no HVMOP_get_mem_type.


Olaf