In case you are using HP ProLiant G6-series or G7-series with Intel-based hardware and you are thinking about virtualization (e.g. booting the hypervisor), you should have a look into this customer advisory. To make it short, the following should work:
# grep GRUB_CMDLINE_XEN /etc/default/grub
GRUB_CMDLINE_XEN="pci=use_crs"
# update-grub
It took me two workdays to discover this solution for a DL180G6. Anyways ... I'm still having trouble on a DL160G6 when booting the hypervisor, suggestions are welcome.