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    SubjectRe: [PATCH v4 0/6] RISC-V Hardware Probing User Interface
    Date
    Am Dienstag, 14. März 2023, 19:32:14 CET schrieb Evan Green:
    >
    > There's been a bunch of off-list discussions about this, including at
    > Plumbers. The original plan was to do something involving providing an
    > ISA string to userspace, but ISA strings just aren't sufficient for a
    > stable ABI any more: in order to parse an ISA string users need the
    > version of the specifications that the string is written to, the version
    > of each extension (sometimes at a finer granularity than the RISC-V
    > releases/versions encode), and the expected use case for the ISA string
    > (ie, is it a U-mode or M-mode string). That's a lot of complexity to
    > try and keep ABI compatible and it's probably going to continue to grow,
    > as even if there's no more complexity in the specifications we'll have
    > to deal with the various ISA string parsing oddities that end up all
    > over userspace.
    >
    > Instead this patch set takes a very different approach and provides a set
    > of key/value pairs that encode various bits about the system. The big
    > advantage here is that we can clearly define what these mean so we can
    > ensure ABI stability, but it also allows us to encode information that's
    > unlikely to ever appear in an ISA string (see the misaligned access
    > performance, for example). The resulting interface looks a lot like
    > what arm64 and x86 do, and will hopefully fit well into something like
    > ACPI in the future.
    >
    > The actual user interface is a syscall, with a vDSO function in front of
    > it. The vDSO function can answer some queries without a syscall at all,
    > and falls back to the syscall for cases it doesn't have answers to.
    > Currently we prepopulate it with an array of answers for all keys and
    > a CPU set of "all CPUs". This can be adjusted as necessary to provide
    > fast answers to the most common queries.

    I've built myself a small test-program [see below], to check the feature
    on the d1-nezha board. Which is how I found the tiny c-extension issue.

    Series works as expected there, so patches 1-4 on a d1-nezha:

    Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>



    hwprobe.c:
    ----------------
    #include <linux/types.h>
    #include <sys/syscall.h>
    #include <stdio.h>
    #include <unistd.h>

    #define __NR_riscv_hwprobe 258

    struct riscv_hwprobe {
    __s64 key;
    __u64 value;
    };

    #define RISCV_HWPROBE_KEY_MVENDORID 0
    #define RISCV_HWPROBE_KEY_MARCHID 1
    #define RISCV_HWPROBE_KEY_MIMPID 2
    #define RISCV_HWPROBE_KEY_BASE_BEHAVIOR 3
    #define RISCV_HWPROBE_BASE_BEHAVIOR_IMA (1 << 0)
    #define RISCV_HWPROBE_KEY_IMA_EXT_0 4
    #define RISCV_HWPROBE_IMA_FD (1 << 0)
    #define RISCV_HWPROBE_IMA_C (1 << 1)
    #define RISCV_HWPROBE_KEY_CPUPERF_0 5
    #define RISCV_HWPROBE_MISALIGNED_UNKNOWN (0 << 0)
    #define RISCV_HWPROBE_MISALIGNED_EMULATED (1 << 0)
    #define RISCV_HWPROBE_MISALIGNED_SLOW (2 << 0)
    #define RISCV_HWPROBE_MISALIGNED_FAST (3 << 0)
    #define RISCV_HWPROBE_MISALIGNED_UNSUPPORTED (4 << 0)
    #define RISCV_HWPROBE_MISALIGNED_MASK (7 << 0)

    int __riscv_hwprobe (struct riscv_hwprobe *pairs, long pair_count,
    long cpu_count, unsigned long *cpus, unsigned long flags)
    {

    return syscall(__NR_riscv_hwprobe, pairs, pair_count, cpu_count, cpus, flags);
    }

    int main(void)
    {
    struct riscv_hwprobe pairs[3];

    pairs[0].key = RISCV_HWPROBE_KEY_MVENDORID;
    pairs[1].key = RISCV_HWPROBE_KEY_MARCHID;
    pairs[2].key = RISCV_HWPROBE_KEY_MIMPID;
    if (__riscv_hwprobe(pairs, 3, 0, NULL, 0) != 0) {
    printf("syscall failed");
    return -1;
    }

    printf("vendorid 0x%x, archid 0x%x, impid 0x%x\n",
    pairs[0].value, pairs[1].value, pairs[2].value);


    pairs[0].key = RISCV_HWPROBE_KEY_CPUPERF_0;
    pairs[1].key = RISCV_HWPROBE_KEY_BASE_BEHAVIOR;
    pairs[2].key = RISCV_HWPROBE_KEY_IMA_EXT_0;
    if (__riscv_hwprobe(&pairs[0], 3, 0, NULL, 0) != 0) {
    printf("syscall failed");
    return -1;
    }

    printf("ima-behavior %d, f+d %d, c %d, misaligned access: %s\n",
    ((pairs[1].value & RISCV_HWPROBE_BASE_BEHAVIOR_IMA) == RISCV_HWPROBE_BASE_BEHAVIOR_IMA),
    ((pairs[2].value & RISCV_HWPROBE_IMA_FD) == RISCV_HWPROBE_IMA_FD),
    ((pairs[2].value & RISCV_HWPROBE_IMA_C) == RISCV_HWPROBE_IMA_C),
    ((pairs[0].value & RISCV_HWPROBE_MISALIGNED_FAST) == RISCV_HWPROBE_MISALIGNED_FAST) ? "fast" : "not-fast"
    );

    return 0;
    }



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