Intel HFI Code Revised For Improving Alder Lake's Hybrid Support On Linux

Back in late 2020 Intel's programming manuals detailed the Enhanced Hardware Feedback Interface for the CPU to provide guidance to the kernel's scheduler on optimal task placement of workloads. While marketed as Thread Director with the new 12th Gen Alder Lake processors, that hardware feedback interface support is getting squared away for the Linux kernel to improve the support for these newest processors.

Microsoft Windows 11 already supports the Intel Hardware Feedback Interface as part of its Alder Lake / Thread Director optimizations while only recently has the Linux kernel support been in the works. In November there were some early "intel_hfi" patches published while right before Christmas Intel dropped a second version of the patches with many changes and improvements stemming from the early code review.

The Intel Hardware Feedback interface is used for communicating performance and power efficiency details about each CPU core to the kernel/OS. The patch series sums it up as:

Currently the Linux kernel relies upon the ITMT / Turbo Boost Max 3.0 driver code with information exposed by the firmware for deciding of proper P and E core handling while the Intel HFI support sounds like it should ultimately be more robust as well as exposing the per-core details to user-space.

The patch series in v2 form of this "intel_hfi" driver is now out for review. Though given the timing and not yet being picked up by the power management's "-next" branch, it's not clear if it will be ready in time for the upcoming Linux 5.17 cycle or be held off until later in 2022.