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.

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https://bugs.launchpad.net/bugs/1685242

Title:
  ovmf hangs at efi with virtio-net memory hotplug

Status in QEMU:
  New

Bug description:
  with qemu 2.9 it hangs at the efi stage when memory-hotplug is enabled
  and it has a virtio-net devices

  the ovmf images where compiled from https://github.com/tianocore/edk2
  (current master)

  reproducer:

  qemu-system-x86_64 -drive
  'if=pflash,unit=0,format=raw,readonly,file=./OVMF_CODE.fd' -drive
  'if=pflash,unit=1,format=raw,file=./my_OVMF_VARS.fd' -smp 1 -vga std
  -netdev 'type=tap,id=mynet' -device 'virtio-net-pci,netdev=mynet'
  -display sdl -nodefaults -m 'size=1G,slots=256,maxmem=1024G'

  interestingly, it works when you do the following:

  - omit the virtio-net-pci device
  - use seabios
  - use less maxmem, e.g. 512G

  qemu was compiled from source (v2.9.0) with following options:

  ./configure --target-list=x86_64-softmmu --disable-xen --enable-gnutls 
--enable-sdl --enable-linux-aio --enable-rbd --enable-libiscsi 
--disable-smartcard --audio-drv-list="alsa" --enable
  -spice --enable-usb-redir --enable-glusterfs --enable-libusb --disable-gtk 
--enable-xfsctl --enable-numa --disable-strip --enable-jemalloc --enable-virtfs 
--disable-libnfs --disable-fdt --disable-guest-agent --disable-guest-agent-msi

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