Benchmarking Retpoline Underflow Protection With Intel Skylake/Kabylake

Beyond the Retpoline support already found in the mainline Linux kernel, developers are working on Retpoline Underflow support that would be used for Intel Skylake and Kabylake CPUs. RETPOLINE_UNDERFLOW protects against falling back to a potentially poisoned indirect branch predictor when a return buffer underflows and this additional protection is needed for Intel Skylake/Kabylake processors. I ran a couple benchmarks...