Ok, I found the messages about trunkating memory in the hypervisor code. And when that happens, as Konrad mentioned, the range can be set to unusable by the hypervisor. It would be good to have a glimpse on the dmesg from a non-xen boot with the same kernel on the same machine. Just to see the E820 layout there and that should also show mtrr coverage.
Ok, I found the messages about trunkating memory in the hypervisor code. And when that happens, as Konrad mentioned, the range can be set to unusable by the hypervisor. It would be good to have a glimpse on the dmesg from a non-xen boot with the same kernel on the same machine. Just to see the E820 layout there and that should also show mtrr coverage.