The Observation Architecture is a set of performance counters for Haswell and newer. Developers interested in all the technical details on these new performance counters can see this public documentation.
Robert Bragg published a new set of patches yesterday for implementing this Observation Architecture in their i915 DRM driver for Haswell. So far this implementation is working with the GPUTop profiling tool and the GL_INTEL_performance_query in Mesa.
The new patches can be found on the dri-devel list while Bragg has also been working on patches for enabling this support under Broadwell too. It's great to see more performance counter work going into the open-source graphics drivers since at one point not too far back the vendors guarded the information much more closely.