RISC-V Hardware Probing Interface¶

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 cpusetsize, 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) with size cpusetsize bytes. For value-like keys (eg. vendor, arch, impl), the returned value will 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 cpusetsize as a shortcut for all online CPUs. The currently supported flags are:

RISCV_HWPROBE_WHICH_CPUS: This flag basically reverses the behavior of sys_riscv_hwprobe(). Instead of populating the values of keys for a given set of CPUs, the values of each key are given and the set of CPUs is reduced by sys_riscv_hwprobe() to only those which match each of the key-value pairs. How matching is done depends on the key type. For value-like keys, matching means to be the exact same as the value. For boolean-like keys, matching means the result of a logical AND of the pair’s value with the CPU’s value is exactly the same as the pair’s value. Additionally, when cpus is an empty set, then it is initialized to all online CPUs which fit within it, i.e. the CPU set returned is the reduction of all the online CPUs which can be represented with a CPU set of size cpusetsize.

All other flags are reserved for future compatibility and must be zero.

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_IMA_V: The V extension is supported, as defined by version 1.0 of the RISC-V Vector extension manual.
- RISCV_HWPROBE_EXT_ZBA: The Zba address generation extension is
  supported, as defined in version 1.0 of the Bit-Manipulation ISA extensions.
- RISCV_HWPROBE_EXT_ZBB: The Zbb extension is supported, as defined
  in version 1.0 of the Bit-Manipulation ISA extensions.
- RISCV_HWPROBE_EXT_ZBS: The Zbs extension is supported, as defined
  in version 1.0 of the Bit-Manipulation ISA extensions.
- RISCV_HWPROBE_EXT_ZICBOZ: The Zicboz extension is supported, as
  ratified in commit 3dd606f (“Create cmobase-v1.0.pdf”) of riscv-CMOs.
- RISCV_HWPROBE_EXT_ZBC The Zbc extension is supported, as defined
  in version 1.0 of the Bit-Manipulation ISA extensions.
- RISCV_HWPROBE_EXT_ZBKB The Zbkb extension is supported, as
  defined in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZBKC The Zbkc extension is supported, as
  defined in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZBKX The Zbkx extension is supported, as
  defined in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZKND The Zknd extension is supported, as
  defined in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZKNE The Zkne extension is supported, as
  defined in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZKNH The Zknh extension is supported, as
  defined in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZKSED The Zksed extension is supported, as
  defined in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZKSH The Zksh extension is supported, as
  defined in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZKT The Zkt extension is supported, as defined
  in version 1.0 of the Scalar Crypto ISA extensions.
- RISCV_HWPROBE_EXT_ZVBB: The Zvbb extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVBC: The Zvbc extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVKB: The Zvkb extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVKG: The Zvkg extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVKNED: The Zvkned extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVKNHA: The Zvknha extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVKNHB: The Zvknhb extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVKSED: The Zvksed extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVKSH: The Zvksh extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZVKT: The Zvkt extension is supported as
  defined in version 1.0 of the RISC-V Cryptography Extensions Volume II.
- RISCV_HWPROBE_EXT_ZFH: The Zfh extension version 1.0 is supported
  as defined in the RISC-V ISA manual.
- RISCV_HWPROBE_EXT_ZFHMIN: The Zfhmin extension version 1.0 is
  supported as defined in the RISC-V ISA manual.
- RISCV_HWPROBE_EXT_ZIHINTNTL: The Zihintntl extension version 1.0
  is supported as defined in the RISC-V ISA manual.
- RISCV_HWPROBE_EXT_ZVFH: The Zvfh extension is supported as
  defined in the RISC-V Vector manual starting from commit e2ccd0548d6c (“Remove draft warnings from Zvfh[min]”).
- RISCV_HWPROBE_EXT_ZVFHMIN: The Zvfhmin extension is supported as
  defined in the RISC-V Vector manual starting from commit e2ccd0548d6c (“Remove draft warnings from Zvfh[min]”).
- RISCV_HWPROBE_EXT_ZFA: The Zfa extension is supported as
  defined in the RISC-V ISA manual starting from commit 056b6ff467c7 (“Zfa is ratified”).
- RISCV_HWPROBE_EXT_ZTSO: The Ztso extension is supported as
  defined in the RISC-V ISA manual starting from commit 5618fb5a216b (“Ztso is now ratified.”)
- RISCV_HWPROBE_EXT_ZACAS: The Zacas extension is supported as
  defined in the Atomic Compare-and-Swap (CAS) instructions manual starting from commit 5059e0ca641c (“update to ratified”).
- RISCV_HWPROBE_EXT_ZICOND: The Zicond extension is supported as
  defined in the RISC-V Integer Conditional (Zicond) operations extension manual starting from commit 95cf1f9 (“Add changes requested by Ved during signoff”)
- RISCV_HWPROBE_EXT_ZIHINTPAUSE: The Zihintpause extension is
  supported as defined in the RISC-V ISA manual starting from commit d8ab5c78c207 (“Zihintpause is ratified”).
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 slower than equivalent byte accesses. Misaligned accesses may be supported directly in hardware, or trapped and emulated by software.
- RISCV_HWPROBE_MISALIGNED_FAST: Misaligned accesses are faster than equivalent byte accesses.
- RISCV_HWPROBE_MISALIGNED_UNSUPPORTED: Misaligned accesses are not supported at all and will generate a misaligned address fault.
RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE: An unsigned int which represents the size of the Zicboz block in bytes.

The Linux Kernel

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RISC-V Hardware Probing Interface¶