OpenSBI Library Usage

OpenSBI provides two types of static libraries:

  1. libsbi.a - A platform-independent generic static library implementing the interface defined by the SBI specifications. Platform-specific processing hooks for the execution of this interface must be provided by the firmware or bootloader linking with this library. This library is installed as <install_directory>/lib/libsbi.a
  2. libplatsbi.a - An example platform-specific static library integrating libsbi.a with platform-specific hooks. This library is available only for the platforms supported by OpenSBI. This library is installed as <install_directory>/platform/<platform_subdir>/lib/libplatsbi.a

Implementations may choose either libsbi.a or libplatsbi.a to link with their firmware or bootloader. In the case of libsbi.a, platform-specific hooks in the form of a struct sbi_platform instance need to be provided.

The platform-specific example firmwares provided by OpenSBI are not mandatory. An implementation may choose to link the OpenSBI generic static library together with an M-mode firmware or bootloader providing the hardware-specific hooks. Since OpenSBI is a statically linked library, users must ensure that the license of these external components is compatible with the OpenSBI license.

Constraints on OpenSBI usage from external firmware

Users have to ensure that an external firmware or bootloader linking against OpenSBI static libraries (libsbi.a or libplatsbi.a) is compiled with the same GCC target options -mabi, -march, and -mcmodel.

There are only two constraints on calling any OpenSBI library function from an external M-mode firmware or bootloader:

  1. The RISC-V MSCRATCH CSR must point to a valid OpenSBI scratch space (i.e. a struct sbi_scratch instance).
  2. The RISC-V SP register (i.e. the stack pointer) must be set per-HART pointing to distinct non-overlapping stacks.

The most important functions from an external firmware or bootloader perspective are sbi_init() and sbi_trap_handler().

In addition to the above constraints, the external firmware or bootloader must ensure that interrupts are disabled in the MSTATUS and MIE CSRs when calling the functions sbi_init() and sbi_trap_handler().

The sbi_init() function should be called by the external firmware or bootloader for each HART that is powered-up at boot-time or in response to a CPU hotplug event.

The sbi_trap_handler() function should be called by the external firmware or bootloader to service the following interrupts and traps:

  1. M-mode timer interrupt
  2. M-mode software interrupt
  3. Illegal instruction trap
  4. Misaligned load trap
  5. Misaligned store trap
  6. Supervisor ecall trap
  7. Hypervisor ecall trap

Note: external firmwares or bootloaders can be more conservative by forwarding all traps and interrupts to sbi_trap_handler().

Definitions of OpenSBI Data Types for the External Firmware

OpenSBI can be built as library using external firmware build system such as EDK2 code base (The open source of UEFI firmware implementation) and linked with external firmware drivers based on the external firmware architecture.

OPENSBI_EXTERNAL_SBI_TYPES identifier is introduced to sbi_types.h for selecting external header file during the build preprocess in order to define OpensSBI data types based on external firmware data type binding. For example, bool is declared as int in sbi_types.h. However, in EDK2 build system, bool is declared as BOOLEAN which is defined as unsigned char data type.

External firmware can define OPENSBI_EXTERNAL_SBI_TYPES in CFLAGS and specify it to the header file maintained in its code tree. However, the external build system has to address the additional include directory for the external header file based on its own build system. For example, -DOPENSBI_EXTERNAL_SBI_TYPES*=OpensbiTypes.h* Above tells sbi_types.h to refer to OpensbiTypes.h instead of using original definitions of data types.