Jeremy Fitzhardinge wrote:

Add the "nosegneg" fake capabilty to the vsyscall page notes. This is
used by the runtime linker to select a glibc version which then
disables negative-offset accesses to the thread-local segment via
%gs. These accesses require emulation in Xen (because segments are
truncated to protect the hypervisor address space) and avoiding them
provides a measurable performance boost.
  

I don't like this because now a kernel compiled with both CONFIG_XEN and 
CONFIG_VMI has "nosegneg" turned on.  We don't actually require this for 
performance or correctness, so it would be nice to be able to 
dynamically turn it off instead of having it forced.

Zach