Thanks,
 
that seems to be the reason. Xen kernel doesn't seem to deal with
sharing of an irq 
An analogue bug is bug 76 
(http://lists.xensource.com/archives/html/xen-bugs/2005-06/msg00074.html
  
  
    Without nousb, do you see sharing of an interrupt line if you 'cat
/proc/interrupts' ?
    
  
  
Dell 2850.(hyperthreading disabled)
with usb a lot of sharing :
  0:     205709          XT-PIC  timer
  1:         84          XT-PIC  i8042
  2:          0          XT-PIC  cascade
  3:        100          XT-PIC  qla6312
  5:         19          XT-PIC  ehci_hcd:usb1
  7:       2828          XT-PIC  megaraid, uhci_hcd:usb4
  8:          0          XT-PIC  rtc
  9:          1          XT-PIC  acpi
 10:          1          XT-PIC  uhci_hcd:usb3
 11:       2014          XT-PIC  uhci_hcd:usb2, eth0
 12:        110          XT-PIC  i8042
 14:       1687          XT-PIC  ide0
megaraid and usb4 ...
Dell 1850 : Just for some extra info  (Hyperthreading enabled) doesn't
seem to have that problem. ( another machine on the netw without xen,
just info) no sharing.
           CPU0       CPU1
  0:  209231690  209354806    IO-APIC-edge  timer
  1:         23         73    IO-APIC-edge  i8042
  8:        661        737    IO-APIC-edge  rtc
  9:          0          0   IO-APIC-level  acpi
 12:         72         70    IO-APIC-edge  i8042
 14:    1881836    1881624    IO-APIC-edge  ide0
169:          0          0   IO-APIC-level  uhci_hcd
177:          0          0   IO-APIC-level  uhci_hcd
185:          0          0   IO-APIC-level  uhci_hcd
193:          0         21   IO-APIC-level  ehci_hcd
201:      39650      56752   IO-APIC-level  megaraid
209:          2        174   IO-APIC-level  qla6312
217:    3799571         80   IO-APIC-level  eth0
NMI:          0          0
LOC:  418586086  418586085
ERR:          0
MIS:          0
  
Here is /proc/interupts from my Dell 1850 without the nousb option (no
usb driver in kernel)