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diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst new file mode 100644 index 000000000000..a52996b22f75 --- /dev/null +++ b/Documentation/arch/riscv/hwprobe.rst @@ -0,0 +1,98 @@ +.. SPDX-License-Identifier: GPL-2.0 + +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 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: + +* :c:macro:`RISCV_HWPROBE_KEY_MVENDORID`: Contains the value of ``mvendorid``, + as defined by the RISC-V privileged architecture specification. + +* :c:macro:`RISCV_HWPROBE_KEY_MARCHID`: Contains the value of ``marchid``, as + defined by the RISC-V privileged architecture specification. + +* :c:macro:`RISCV_HWPROBE_KEY_MIMPLID`: Contains the value of ``mimplid``, as + defined by the RISC-V privileged architecture specification. + +* :c:macro:`RISCV_HWPROBE_KEY_BASE_BEHAVIOR`: A bitmask containing the base + user-visible behavior that this kernel supports. The following base user ABIs + are defined: + + * :c:macro:`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). + +* :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0`: A bitmask containing the extensions + that are compatible with the :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`: + base system behavior. + + * :c:macro:`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. + + * :c:macro:`RISCV_HWPROBE_IMA_C`: The C extension is supported, as defined + by version 2.2 of the RISC-V ISA manual. + + * :c:macro:`RISCV_HWPROBE_IMA_V`: The V extension is supported, as defined by + version 1.0 of the RISC-V Vector extension manual. + + * :c:macro:`RISCV_HWPROBE_EXT_ZBA`: The Zba address generation extension is + supported, as defined in version 1.0 of the Bit-Manipulation ISA + extensions. + + * :c:macro:`RISCV_HWPROBE_EXT_ZBB`: The Zbb extension is supported, as defined + in version 1.0 of the Bit-Manipulation ISA extensions. + + * :c:macro:`RISCV_HWPROBE_EXT_ZBS`: The Zbs extension is supported, as defined + in version 1.0 of the Bit-Manipulation ISA extensions. + +* :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A bitmask that contains performance + information about the selected set of processors. + + * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNKNOWN`: The performance of misaligned + accesses is unknown. + + * :c:macro:`RISCV_HWPROBE_MISALIGNED_EMULATED`: Misaligned accesses are + emulated via software, either in or below the kernel. These accesses are + always extremely slow. + + * :c:macro:`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. + + * :c:macro:`RISCV_HWPROBE_MISALIGNED_FAST`: Misaligned accesses are faster + than equivalent byte accesses. + + * :c:macro:`RISCV_HWPROBE_MISALIGNED_UNSUPPORTED`: Misaligned accesses are + not supported at all and will generate a misaligned address fault. |