Here's an example use of the vsyscall via LD_PRELOAD.

Attached is an example library that can be LD_PRELOADED to alias glib's
gettimeofday function w/ the vsyscall implementation. I've also included
a quick performance test to give a rough idea of the savings this gives.

Example run on a SMP P4 box:

[jstultz@elm3b52 vsyscall_test]$ ./run.sh 
Normal gettimeofday
gettimeofday ( 1391621us / 1000000runs ) = 1.391620us
vsyscall LD_PRELOAD gettimeofday
gettimeofday ( 286567us / 1000000runs ) = 0.286567us

So it looks like a pretty big win.

thanks
-john