Xen users - Apr 2012 - Xen-4.1 : Serial port from domU ?

Hello,

With Xen-4, is it still possible to access some serial device 
(an UPS in my case) from a domU ?

I can''t get it to work : ttyS0 is here in dom0 ( as shown by
commands like grep serial /proc/ioports ; dmesg | grep ttyS ; 
stty -aF /dev/ttyS0 ) but not from a domU : 

$ grep serial /proc/ioports

$ dmesg | grep ttyS

$ stty -aF /dev/ttyS0
stty: /dev/ttyS0: Input/output error

$ ls -l /dev/ttyS0
crw-rw----. 1 root dialout 4, 64 Apr 12 04:46 /dev/ttyS0

Is there any thing to do in addition to the Xen-3 usual 
configuration :
irq = [ 4 ]
ioports = [ "3f8-3ff" ]

Thanks,

-- 
Philippe Naudin