tap:qcow causes dom-U to hang in 3.0.3

[Roland Paterson-Jones <roland@rolandpj.com>]

Hi all

I have had some success with the new blktap support in Xen 3.0.3. In 
particular, the loopback drop-in, tap:aio: works somewhat reliably.

However, I am really interested in copy-on-write support, and hence have 
been trying tap:qcow:. This has had limited success, in that dom-U's 
appear to launch. However, as soon as I try to console or ssh into the 
dom-U's they apparently freeze, and I can get no further with them.

I have managed to 'xm terminate' the frozen dom-U's, but this leaves 
xend in some inconsistent state. All further 'xm xxx' commands barf with 
the following error:

[root@dom0]# xm list
Error: Device 2050 not connected
Usage: xm list [options] [Domain, ...]

Curiously, it seems that device 2050 was a secondary block device 
exposed using phy:.

So, any advice on how to debug this?

Thanks and kind regards
Roland