If after booting your vm, you realize that you forget to add a special block from dom0, it is still possible to add it when your vm are running. If you read the man page of xen :
# man xm
you will see that you need to do that :
# xm block-attach our_domU_id phy:/dev/hda /dev/sda rw
to add the real block device :
from dom0 to a domU. But it doesn't work actually, and you will get this error :
Error: Invalid mode Usage: xm block-attach <Domain> <BackDev> <FrontDev> <Mode> Create a new virtual block device.
Strange, as the mode in the man page did say :
Actually, you have to specify another mode, w instead of rw, and r instead of ro. So, you have to type this :
# xm block-attach our_domU_id phy:/dev/hda /dev/sda w
and it will work.