Comment 13 for bug 1685242

Last night I remembered another tidbit:

In OVMF, you can entirely disable the 64-bit MMIO aperture.

  -fw_cfg name=opt/ovmf/X-PciMmio64Mb,string=0

If you use the above switch, the firmware log should contain the following message:

> GetFirstNonAddress: disabling 64-bit PCI host aperture

This will force the BAR allocations into 32-bit address space. Obviously, that makes it a lot easier to run out of the (much smaller) 32-bit MMIO aperture, but if you don't have PCI(E) devices with large MMIO BARs, then it should work (e.g. it should totally work in your present use case).

And then you should be able to increase the DIMM hotplug area significantly, without running out of the GPA range covered by 39 physical address bits:

    //
    // There's nothing more to do; the amount of memory above 4GB fully
    // determines the highest address plus one. The memory hotplug area (see
    // below) plays no role for the firmware in this case.
    //

"This case" beeing (Pci64Size == 0).

Hope this helps.