Yes, this is the PCI hole. I've got the same issue with a i945.
Despite the claim that the motherboard "supports" 4Gb and using a
EM64T CPU,
768M were stolen for PCI. By playing with BIOS options, disabling
all unused
devices I trimmed this down to ~600M.
In order to really use 4GB you need a i955X chipset.
I have an i965 chipset and I also lose 768MB of 4GB to the PCI hole
(it's sort of ok, at least I was aware of it when deploying this
machine).
With a 32bits or 64bits OS ? with 32bits, you won't see more than 4GB
including the PCI hole anyway, even if the chipset supports
remaming the
remaining RAM above 4GB.
However, my question is what would happen if I add more memory to the
machine. I.e. if I boost it to 8GB would I still only get 3.25GB for
Xen or would I get 7.25GB not having to bother about the hole in the
middle?
NetBSD/Xen is still 32bits, so it's limited to 4GB.