Hi,
well, I've just successfully installed RHEL5.11 and RHEL6.10 from i386 ISOs on virtio root disk + 64Gb RAM (upstream QEMU), and they booted to command prompt without any issues.
Oh, interesting. Apparently the physical address space is the same in 32-bit and long mode, i.e. on x86-64 hardware PAE paging mode can map physical addresses above 64G.
rhel-6-32 kraxel ~# uname -a Linux rhel-6-32.lab.home.kraxel.org 2.6.32-754.35.1.el6.i686 #1 SMP Wed Sep 16 06:48:16 EDT 2020 i686 i686 i386 GNU/Linux rhel-6-32 kraxel ~# cat /proc/iomem [ ... ] fffc0000-ffffffff : reserved 100000000-17fffffff : System RAM 7000000000-77ffffffff : PCI Bus 0000:00 7000000000-7000003fff : 0000:00:02.0 7000004000-7000007fff : 0000:00:03.0 7000004000-7000007fff : virtio-pci [ ... ]
So the 64G / 36 phys-bits apparently simply happens to be the limit implemented by 32-bit processors (and early 64-bit processors from intel too). It's NOT a hard limit for PAE paging mode.
So I'm wondering why the 32-bit guests are running into trouble in the first place?
Also using the 64G as indicator for the heuristics doesn't look that useful any more ...
take care, Gerd