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diff --git a/Documentation/x86/intel-hfi.rst b/Documentation/x86/intel-hfi.rst deleted file mode 100644 index 49dea58ea4fb..000000000000 --- a/Documentation/x86/intel-hfi.rst +++ /dev/null @@ -1,72 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0 - -============================================================ -Hardware-Feedback Interface for scheduling on Intel Hardware -============================================================ - -Overview --------- - -Intel has described the Hardware Feedback Interface (HFI) in the Intel 64 and -IA-32 Architectures Software Developer's Manual (Intel SDM) Volume 3 Section -14.6 [1]_. - -The HFI gives the operating system a performance and energy efficiency -capability data for each CPU in the system. Linux can use the information from -the HFI to influence task placement decisions. - -The Hardware Feedback Interface -------------------------------- - -The Hardware Feedback Interface provides to the operating system information -about the performance and energy efficiency of each CPU in the system. Each -capability is given as a unit-less quantity in the range [0-255]. Higher values -indicate higher capability. Energy efficiency and performance are reported in -separate capabilities. Even though on some systems these two metrics may be -related, they are specified as independent capabilities in the Intel SDM. - -These capabilities may change at runtime as a result of changes in the -operating conditions of the system or the action of external factors. The rate -at which these capabilities are updated is specific to each processor model. On -some models, capabilities are set at boot time and never change. On others, -capabilities may change every tens of milliseconds. For instance, a remote -mechanism may be used to lower Thermal Design Power. Such change can be -reflected in the HFI. Likewise, if the system needs to be throttled due to -excessive heat, the HFI may reflect reduced performance on specific CPUs. - -The kernel or a userspace policy daemon can use these capabilities to modify -task placement decisions. For instance, if either the performance or energy -capabilities of a given logical processor becomes zero, it is an indication that -the hardware recommends to the operating system to not schedule any tasks on -that processor for performance or energy efficiency reasons, respectively. - -Implementation details for Linux --------------------------------- - -The infrastructure to handle thermal event interrupts has two parts. In the -Local Vector Table of a CPU's local APIC, there exists a register for the -Thermal Monitor Register. This register controls how interrupts are delivered -to a CPU when the thermal monitor generates and interrupt. Further details -can be found in the Intel SDM Vol. 3 Section 10.5 [1]_. - -The thermal monitor may generate interrupts per CPU or per package. The HFI -generates package-level interrupts. This monitor is configured and initialized -via a set of machine-specific registers. Specifically, the HFI interrupt and -status are controlled via designated bits in the IA32_PACKAGE_THERM_INTERRUPT -and IA32_PACKAGE_THERM_STATUS registers, respectively. There exists one HFI -table per package. Further details can be found in the Intel SDM Vol. 3 -Section 14.9 [1]_. - -The hardware issues an HFI interrupt after updating the HFI table and is ready -for the operating system to consume it. CPUs receive such interrupt via the -thermal entry in the Local APIC's Local Vector Table. - -When servicing such interrupt, the HFI driver parses the updated table and -relays the update to userspace using the thermal notification framework. Given -that there may be many HFI updates every second, the updates relayed to -userspace are throttled at a rate of CONFIG_HZ jiffies. - -References ----------- - -.. [1] https://www.intel.com/sdm |