Re: Idea: Small Address Spaces

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

Jacob Gorm Hansen wrote:
> hi,
> 
> in the cases where users (like me) wish to run Xen with MPI on Ethernets 
> or similar, and don't care too much about driver isolation, I am 
> thinking of trying to apply Jochen Liedtke's old 'small address spaces' 
> hack, to see if I can improve domU I/O performance.
> 
> My idea is to reserve some additional virtual address space below Xen, 
> e.g. at 0xF0000000, and map the kernel part of dom0* there permanently. 
> The user space part of dom0 I would map as normal from 0 - 0xC0000000, 
> to avoid relinking dom0 applications.  I would use the segments to keep 
> the domUs below 0xF0000000. In this way, TLB flushes should only be 
> necessary when dom0 exits to user space, not when handling interrupts or 
> when domUs are asking for I/O.

Hmmm turns out this is pretty hard to do without relinking userspace, as 
naturally the linux0 likes to peek and poke user-space addresses itself, 
as does Xen.

I guess what I could do (my original plan actually, before I got too 
clever for my own good), would be to squeeze userspace into the 
permanent mapping as well, and then have a relinked busybox or similar 
in there, instead of a full, standard Linux disto.

Jacob