There are tons of virtualization projects based on tracing: user-mode linux, umview/kmview, fakeroot-ng, cde.
Ptrace is ugly and old but fundamentals tools rely on it: gdb, strace. New tracing facilities like uprobes, lttng and systemtap have been designed mainly (only) for debugging, not for virtualization. In the meanwhile there are new efforts to create more flexible kernels: anykernels and millikernels. These new approaches need a specific support: hooks, hypercall or tracing. It does not matter how we call it. Utrace is (was) an option, with some minor effort at kernel level, even classic ptrace could be implemented at user level for back compatibility.
The workshop wants to be a technologically agnostic discussion on tracing supports for virtualization and for new flexible kernels.
All contributors of new ideas are welcome to the open discussion.