Known limitations and work in progress
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The "xenctl" tool used for controling domains is still rather clunky
and not very user friendly. In particular, it should have an option to
create and start a domain with all the necessary parameters set from a
named xml file.

The java xenctl tool is really just a frontend for a bunch of C tools
named xi_* that do the actual work of talking to Xen and setting stuff
up. Some local users prefer to drive the xi_ tools directly, typically
from simple shell scripts. These tools are even less user friendly
than xenctl but its arguably clearer what's going on.

There's also a nice web based interface for controlling domains that
uses apache/tomcat. Unfortunately, this has fallen out of sync with
respect to the underlying tools, so is currently not built by default
and needs fixing. 

The current Virtual Firewall Router (VFR) implementation in the
snapshot tree is very rudimentary, and in particular, lacks the IP
port-space sharing across domains that we've proposed that promises to
provide a better alternative to NAT.  There's a complete new
implementation under development which also supports much better
logging and auditing support.  The current network scheduler is just
simple round-robin between domains, without any rate limiting or rate
guarantees. Dropping in a new scheduler should be straightforward, and
is planned as part of the VFRv2 work package.

Another area that needs further work is the interface between Xen and
domain0 user space where the various XenoServer control daemons run.
The current interface is somewhat ad-hoc, making use of various
/proc/xeno entries that take a random assortment of arguments. We
intend to reimplement this to provide a consistent means of feeding
back accounting and logging information to the control daemon. Also,
we should provide all domains with a read/write virtual console
interface -- currently for domains >1 it is output only.

There's also a number of memory management hacks that didn't make this
release: We have plans for a "universal buffer cache" that enables
otherwise unused system memory to be used by domains in a read-only
fashion. We also have plans for inter-domain shared-memory to enable
high-performance bulk transport for cases where the usual internal
networking performance isn't good enough (e.g. communication with a
internal file server on another domain).

We also have plans to implement domain suspend/resume-to-file. This is
basically an extension to the current domain building process to
enable domain0 to read out all of the domain's state and store it in a
file. There are complications here due to Xen's para-virtualised
design, whereby since the physical machine memory pages available to
the guest OS are likely to be different when the OS is resumed, we
need to re-write the page tables appropriately. 

We have the equivalent of balloon driver functionality to control
domain's memory usage, enabling a domain to give back unused pages to
Xen. This needs properly documenting, and perhaps a way of domain0
signalling to a domain that it requires it to reduce its memory
footprint, rather than just the domain volunteering.

The current disk scheduler is rather simplistic (batch round robin),
and could be replaced by e.g. Cello if we have QoS isolation
problems. For most things it seems to work OK, but there's currently
no service differentiation or weighting.

Currently, although Xen runs on SMP and SMT (hyperthreaded) machines,
the scheduling is far from smart -- domains are currently statically
assigned to a CPU when they are created (in a round robin fashion).
The scheduler needs to be modified such that before going idle a
logical CPU looks for work on other run queues (particularly on the
same physical CPU). 

Xen currently only supports uniprocessor guest OSes. We have designed
the Xen interface with MP guests in mind, and plan to build an MP
Linux guest in due course. Basically, an MP guest would consist of
multiple scheduling domains (one per CPU) sharing a single memory
protection domain. The only extra complexity for the Xen VM system is
ensuring that when a page transitions from holding a page table or
page directory to a write-able page, we must ensure that no other CPU
still has the page in its TLB to ensure memory system integrity.  One
other issue for supporting MP guests is that we'll need some sort of
CPU gang scheduler, which will require some research.