Comment 11 for bug 1685242

The reason why it behaves differently with machine types <= 2.4 is that
- up to and including qemu-2.4, the calculation of the DIMM hotplug area was incorrect,
- in 2.5, the calculation was fixed,
- but for the migration compatibility of machine types <= 2.4, the old calculation was preserved for those machine types (the DIMM hotplug area is guest-visible)

See the following (adjacent) commits:
- 3385e8e2640e ("pc: memhotplug: fix incorrectly set reserved-memory-end", 2015-09-10)
- 2f8b50083b32 ("pc: memhotplug: keep reserved-memory-end broken on 2.4 and earlier machines", 2015-09-10)

These commits were first released in v2.5.0.

I'll look at your logs now.