Small address spaces update

[Jacob Gorm Hansen <jacobg@diku.dk>]

hi,

I have managed to get the 'small address spaces' hack running in 
xen-unstable. I have relinked my dom0 userspace (busybox + vmtools) and 
kernel to share the range 0xf0000000 - 0xfc000000, and my domU kernel to 
stay below 0xf0000000, on x86-32.

I use the top part of the domU page directories as a cache for the 
currently running I/O-domain (right now I only have one, dom0), and so 
unless I need to update this cache I can context switch without flushing 
the TLB. This saves more 99% af TLB flushes when doing simple I/O-bound 
stuff such as running iPerf inside a domU.

In some cases (when dom0 has updated or switched its page directory) I 
need to update the cache in the domU page directory, that involves doing 
a memcpy of 48*4 = 192 bytes. How frequently this happens depends on the 
kind of stuff running in dom0 userspace, i.e. for the iperf test is very 
rare.

At this point I have no idea if this increases domU I/O performance, as 
right now I am getting bad iperf numbers in domU both with and without 
this patch applied to Xen, but just wanted to let people know that I am 
still working on this thing when I can find the time.

Best regards,
Jacob