Known limitations and work in progress
======================================

The current Xen Virtual Firewall Router (VFR) implementation in the
snapshot tree is very rudimentary, and in particular, lacks the RSIP
IP port-space sharing across domains that provides a better
alternative to NAT.  There's a complete new implementation under
development which also supports much better logging and auditing
support. For now, if you want NAT, see the xen_nat_enable scripts and
get domain0 to do it for you.

There are also a number of memory management enhancements 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 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 (see section on
the improved control interface).

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.

Currently, the privileged domain0 can request access to the underlying
hardware. This is how we enable the VGA console and Xserver to run in
domain0. We are planning on extending this functionality to enable
other device drivers for 'low performance' devices to be run in
domain0, and then virtualized to other domains by domain0. This will
enable random PCMCIA and USB devices to be used that we're unlikely to
ever get around to writing a Xen driver for.

We'd also like to experiment moving the network and block device
drivers out of Xen, and each into their own special domains that are
given access to the specific set of h/w resources they need to
operate.  This will provide some isolation against faulty device
drivers, potentially allowing them to be restarted on failure. There
may be more context switches incurred, but due to Xen's pipelined
asynchronous i/o interface we expect this overhead to be amortised.
This architecture would also allow device drivers to be easily
upgraded independent of Xen, which is necessary for our vision of Xen
as a next-gen BIOS replacement.