Multiple Processor (MP) Initialization

This section is intended to document the purpose of performing multiprocessor initialization and its possible ways in coreboot space.

Entire CPU multiprocessor initialization can be divided into two parts

  1. BSP (Boot Strap Processor) Initialization
  2. AP (Application Processor) Initialization

Problem Statement

  1. coreboot is capable enough to handle multiprocessor initialization on IA platforms.
  2. With restricted CPU programming logic, there might be some cases where certain feature programming can’t be open sourced at early development of SOC.

Platform code might need to compromise on those closed source nature of CPU programming if we don’t plan to provide an alternate interface which can be used by coreboot to get rid of such close sourced CPU programming.

Possible Solution Space

Considering these facts, there are 3 possible solutions to perform MP initialization from coreboot + FSP space.

  1. coreboot to perform complete MP initialization by its own. This includes BSP and AP programming of CPU features mostly non-restricted one. This is the default configuration. Most SoCs like SKL, KBL, APL are okay to make use of this MP initialization method.
  2. Alternatively, SoC users also can skip coreboot doing MP initialization and make use of FSP binary to perform same task. This can be achieved by using Kconfig name USE_INTEL_FSP_MP_INIT. As of 2019 all Google Chrome products are using coreboot native MP initialization mechanism and some IOTG platforms are using FSP MP Init solution as well.
  3. Final option is to let coreboot publish PPI (PEIM to PEIM Interface) to perform some restricted (closed source) CPU programming. In that case, coreboot will use its native MP init and additionally publish MP service PPI for FSP to consume. FSP will execute some CPU programming using same PPI service from its own context. One can use USE_INTEL_FSP_TO_CALL_COREBOOT_PUBLISH_MP_PPI Kconfig to perform this operation.

For latest SoCs like CNL, WHL, ICL, etc, its recommended to make use of this option in order to perform SGX and C6DRAM enabling.

Typically all platforms supported by FSP 2.1 specification will have external PPI service feature implemented.

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