This most curious patch allows the fixmap on i386 to be unfixed. The result is that we can create a dynamically sizable hole at the top of kernel linear address space. I know at least some virtualization developers are interested in being able to achieve this to achieve run-time sizing of a hole in which a hypervisor can live, or at least to test out the performance characteristics of different sized holes. I have not run any performance numbers yet to see how much the cost of making this dynamic affects native performance, but again I would stress this is a highly experimental patch and I am looking for feedback and any performance data from other systems that people are kind enough to share. I'm not advocating that this get pushed into the mainline Linux tree at this point by any means! I believe at least the Xen folks would be interested in playing around with this for experimenting with different MPT and frame table sizes for PAE support in a way that doesn't require recompiling the Linux guest each time - if the performance impact proves to be negligble, this gives a lot of flexibility to any virtual machine which runs a hypervisor aware kernel. Although I did as much as possible to make the vsyscall relocation appear clean to userspace, I can't guarantee this patch won't set fire to your chair and electrocute your cat. Please move all pets to a safe location before attempting to use this. Zachary Amsden