Crate rustsbi

Expand description

A minimal RISC-V’s SBI implementation library in Rust.

What is RISC-V SBI?

RISC-V SBI is short for RISC-V Supervisor Binary Interface. SBI acts as a bootloader environment to your operating system kernel. A SBI implementation will bootstrap your kernel, and provide an environment when your kernel is running.

More generally, The SBI allows supervisor-mode (S-mode or VS-mode) software to be portable across all RISC-V implementations by defining an abstraction for platform (or hypervisor) specific functionality.

How to use RustSBI in your supervisor software

SBI features include boot sequence and a kernel environment. To bootstrap your kernel, place kernel into RustSBI implementation defined address, then RustSBI will prepare an environment and jump to this address.

Make SBI environment calls

To use the kernel environment, you either use SBI calls or emulated instructions. SBI calls are similar to operating systems’ syscalls. RISC-V SBI defined many SBI modules, and in each module there are different functions, you should pick a function before calling. Then, you should prepare some parameters, whose definition are not the same among functions.

Now you have a module number, a function number, and a few SBI call parameters. You invoke a special ecall instruction on supervisor level, and it will trap into machine level SBI implementation. It will handle your ecall, similar to your kernel handling system calls from user level.

SBI functions return two values other than one. First value will be an error number, it will tell if SBI call have succeeded, or which error have occurred. Second value is the real return value, its meaning is different according to which function you calls.

Call SBI in different programming languages

Making SBI calls are similar to making system calls.

Module number is required to put on register a7, function number on a6. Parameters should be placed from a0 to a5, first into a0, second into a1, etc. Unused parameters can be set to any value or leave untouched.

After registers are ready, invoke an instruction called ecall. Then, the return value is placed into a0 and a1 registers. The error value could be read from a0, and return value is placed into a1.

In Rust, here is an example to call SBI functions using inline assembly:

#[inline(always)]
fn sbi_call(extension: usize, function: usize, arg0: usize, arg1: usize) -> SbiRet {
    let (error, value);
    match () {
        #[cfg(any(target_arch = "riscv32", target_arch = "riscv64"))]
        () => unsafe { asm!(
            "ecall",
            in("a0") arg0, in("a1") arg1,
            in("a6") function, in("a7") extension,
            lateout("a0") error, lateout("a1") value,
        ) },
        #[cfg(not(any(target_arch = "riscv32", target_arch = "riscv64")))]
        () => {
            drop((extension, function, arg0, arg1));
            unimplemented!("not RISC-V instruction set architecture")
        }
    };
    SbiRet { error, value }
}

#[inline]
pub fn get_spec_version() -> SbiRet {
    sbi_call(EXTENSION_BASE, FUNCTION_BASE_GET_SPEC_VERSION, 0, 0)
}

Complex SBI functions may fail. In this example we only take the value, but in complete designs we should handle the error value returned from SbiRet.

You may use other languages to call SBI environment. In C programming language, we can call like this:

#define SBI_CALL(module, funct, arg0, arg1, arg2, arg3) ({ \
    register uintptr_t a0 asm ("a0") = (uintptr_t)(arg0); \
    register uintptr_t a1 asm ("a1") = (uintptr_t)(arg1); \
    register uintptr_t a2 asm ("a2") = (uintptr_t)(arg2); \
    register uintptr_t a3 asm ("a3") = (uintptr_t)(arg3); \
    register uintptr_t a7 asm ("a6") = (uintptr_t)(funct); \
    register uintptr_t a7 asm ("a7") = (uintptr_t)(module); \
    asm volatile ("ecall" \
        : "+r" (a0), "+r" (a1) \
        : "r" (a1), "r" (a2), "r" (a3), "r" (a6), "r" (a7) \
        : "memory") \
    {a0, a1}; \
})

#define SBI_CALL_0(module, funct) SBI_CALL(module, funct, 0, 0, 0, 0)

static inline sbiret get_spec_version() {
    SBI_CALL_0(EXTENSION_BASE, FUNCTION_BASE_GET_SPEC_VERSION)
}

Non-features

RustSBI is designed to strictly adapt to the RISC-V Supervisor Binary Interface specification. Other features useful in developing kernels and hypervisors maybe included in other Rust ecosystem crates other than this package.

Hardware discovery and feature detection

According to the RISC-V SBI specification, SBI does not specify any method for hardware discovery. The supervisor software must rely on the other industry standard hardware discovery methods (i.e. Device Tree or ACPI) for that.

To detect any feature under bare metal or under supervisor level, developers may depend on any hardware discovery methods, or use try-execute-trap method to detect any instructions or CSRs. If SBI is implemented in user level emulators, it may requires to depend on operating system calls or use the signal trap method to detect any RISC-V core features.

Where can I get RustSBI binary file for XX platform?

RustSBI is designed to be a library instead of providing any binary files to specific platforms. Chip or board manufacturers should provide their own SBI implementation project using RustSBI as a dependency.

The RustSBI team provides reference implementation for several platforms, but they are for evaluation only and should not be used in production. RustSBI itself cannot decide for all arbitrary users, so developers are encouraged to use RustSBI as a Rust crate dependency to support their own SBI implementation, other than use reference implementation directly when in production. SBI feature demands are different among users, one feature would be useful for this user, but it will be considered not useful and takes up lots of flash room for other users.

RustSBI is not designed to include all platforms available in official repository. For an actual platform users may consult board or SoC manufacturer other than RustSBI repository itself.

The reason to that is that if some repository includes all platforms it support, there could be lots of non technical reasons that will bar one or a certain group of developers from merging their code into main or upstream branches. For example, if a draft version of actual platform is produced, it will mean to write for one draft version as well as long as this software is under maintenance. As software developer may not want to write for it, it’s better to include minimal feature in core repository, and leave other features for downstream developers.

Notes for RustSBI developers

This library adapts to embedded Rust’s embedded-hal crate to provide basic SBI features. When building for own platform, implement traits in this library and pass them to the functions begin with init. After that, you may call rustsbi::ecall in your own exception handler which would dispatch parameters from supervisor to the traits to execute SBI functions.

The library also implements useful functions which may help with platform specific binaries. The enter_privileged maybe used to enter the operating system after the initialization process is finished. The LOGO should be printed if necessary when the binary is initializing.

Note that this crate is a library which contains common building blocks in SBI implementation. It is not intended to be used directly; users should build own platforms with this library. RustSBI provides implementations on common platforms in separate platform crates.