Linux question, user space virtual mem layout

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

hi,

I am trying to allow dom0 to be mapped permanently in the top parts of 
all domU address spaces, to save some TLB flushes when doing I/O (on 
x86-32) and see how much that helps performance.

So far I have managed to get the dom0 kernel, and a relinked user space, 
to live between 0xf0000000 and 0xfc000000 where Xen starts. But I have 
the problem that even though me user-space binaries are linked to reside 
in the 0xf0000000 -> 0xf6000000 virtual area, mmap() in Linux still 
likes to create mappings below 0xf0000000.

Can anyone on this list tell me how I can instruct Linux not to go below 
a certain address when dealing out virtual mappings to user-space?

Thanks in advance,
Jacob