Xen users - Jul 2014 - No USB keyboard / mouse when booted to Xen on newer Kernels

Hi,

I tried to use Xen 4.1.6 on kernel 3.11 (from Ubuntu 12.04) and Xen
4.4 on kernel 3.13 (from Ubuntu 14.04), but the USB keyboard and mouse
failed to work. As no USB keyboard, I could not get xm dmesg log but
the boot sequence showed something like "....ehci-pci error -110".

When booted to normal kernel without Xen, the USB keyboard and mouse
worked again.

I hope someone could advise how to make USB keyboard / mouse to work
with Xen 4.1.6 on kernel 3.11, and Xen 4.4 on kernel 3.13 (from Ubuntu
14.04).

Thanks a lot.

John Mok