RISC-V Hardware Probing InterfaceΒΆ

The RISC-V hardware probing interface is based around a single syscall, which is defined in <asm/hwprobe.h>:

struct riscv_hwprobe {
    __s64 key;
    __u64 value;

long sys_riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
                       size_t cpu_count, cpu_set_t *cpus,
                       unsigned int flags);

The arguments are split into three groups: an array of key-value pairs, a CPU set, and some flags. The key-value pairs are supplied with a count. Userspace must prepopulate the key field for each element, and the kernel will fill in the value if the key is recognized. If a key is unknown to the kernel, its key field will be cleared to -1, and its value set to 0. The CPU set is defined by CPU_SET(3). For value-like keys (eg. vendor/arch/impl), the returned value will be only be valid if all CPUs in the given set have the same value. Otherwise -1 will be returned. For boolean-like keys, the value returned will be a logical AND of the values for the specified CPUs. Usermode can supply NULL for cpus and 0 for cpu_count as a shortcut for all online CPUs. There are currently no flags, this value must be zero for future compatibility.

On success 0 is returned, on failure a negative error code is returned.

The following keys are defined:

  • RISCV_HWPROBE_KEY_MVENDORID: Contains the value of mvendorid, as defined by the RISC-V privileged architecture specification.

  • RISCV_HWPROBE_KEY_MARCHID: Contains the value of marchid, as defined by the RISC-V privileged architecture specification.

  • RISCV_HWPROBE_KEY_MIMPLID: Contains the value of mimplid, as defined by the RISC-V privileged architecture specification.

  • RISCV_HWPROBE_KEY_BASE_BEHAVIOR: A bitmask containing the base user-visible behavior that this kernel supports. The following base user ABIs are defined:

    • RISCV_HWPROBE_BASE_BEHAVIOR_IMA: Support for rv32ima or rv64ima, as defined by version 2.2 of the user ISA and version 1.10 of the privileged ISA, with the following known exceptions (more exceptions may be added, but only if it can be demonstrated that the user ABI is not broken):

      • The :fence.i: instruction cannot be directly executed by userspace programs (it may still be executed in userspace via a kernel-controlled mechanism such as the vDSO).

  • RISCV_HWPROBE_KEY_IMA_EXT_0: A bitmask containing the extensions that are compatible with the RISCV_HWPROBE_BASE_BEHAVIOR_IMA: base system behavior.

    • RISCV_HWPROBE_IMA_FD: The F and D extensions are supported, as defined by commit cd20cee ("FMIN/FMAX now implement minimumNumber/maximumNumber, not minNum/maxNum") of the RISC-V ISA manual.

    • RISCV_HWPROBE_IMA_C: The C extension is supported, as defined by version 2.2 of the RISC-V ISA manual.

  • RISCV_HWPROBE_KEY_CPUPERF_0: A bitmask that contains performance information about the selected set of processors.

    • RISCV_HWPROBE_MISALIGNED_UNKNOWN: The performance of misaligned accesses is unknown.

    • RISCV_HWPROBE_MISALIGNED_EMULATED: Misaligned accesses are emulated via software, either in or below the kernel. These accesses are always extremely slow.

    • RISCV_HWPROBE_MISALIGNED_SLOW: Misaligned accesses are supported in hardware, but are slower than the cooresponding aligned accesses sequences.

    • RISCV_HWPROBE_MISALIGNED_FAST: Misaligned accesses are supported in hardware and are faster than the cooresponding aligned accesses sequences.

    • RISCV_HWPROBE_MISALIGNED_UNSUPPORTED: Misaligned accesses are not supported at all and will generate a misaligned address fault.