Thomas-Bjorkman I have got some answer for you, I will post them in the same order you presented them.
This debug technology is called Intel® Processor Trace for Intel® Processors;
1. Where to find documents om Hardware assisted Trace?
Intel® Processor Trace documentation can be found in Datasheets for the exact processor platform and in the Intel Confidential Materials available only under NDA.
2. Does HAT requires added HW?
Intel® PT is a new tracing capability added to Intel Architecture, for use in software debug and profiling. No additional HW required.
3. What SW interface to read out trace?
Intel VTune™ Amplifier for Systems and the Intel System Debugger are part of Intel System Studio 2015 (https://software.intel.com/intel-system-studio), which includes updates for new debug and trace features on this latest platform, including Intel PT and Intel Trace Hub. Linux perf utility, with support for Intel PT, is available for download at https://github.com/virtuoso/linux-perf/tree/intel_pt. It requires rebuilding the kernel and the perf utility. ASSET InterTech SourcePoint for Intel, Lauterbach* TRACE32*, etc. Or you can try Intel® Platform Analysis Library (Intel® PAL) https://software.intel.com/intel-platform-analysis-library provides libraries for trace decode and control flow reconstruction, to ease development of Intel® PTenabled tools.
4. What can be read out from trace?
Precise software control flow and timing information. This provides enhanced ability to debug software crashes, hangs, or other anomalies, as well as responsiveness and short-duration performance issues. (including Map linear-address to image files, Map CR3 value to application, Log module load/unload and JIT info), etc.
5. Can trace be configured to filter certain evens?
Can filter trace by CPL, CR3, or IP ranges, VMM can opt-out of tracing, SMM & SGX filtered out by default, can opt-in.
The answers above are general and based on the common platforms capabilities. Exact features and details depend on the exact platform.