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    Subject[PATCH v5 0/6] RISC-V Hardware Probing User Interface
    Date

    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.

    An example series in glibc exposing this syscall and using it in an
    ifunc selector for memcpy can be found at [1]. I'm about to send a v2
    of that series out that incorporates the vDSO function.

    I was asked about the performance delta between this and something like
    sysfs. I created a small test program [2] and ran it on a Nezha D1
    Allwinner board. Doing each operation 100000 times and dividing, these
    operations take the following amount of time:
    - open()+read()+close() of /sys/kernel/cpu_byteorder: 3.8us
    - access("/sys/kernel/cpu_byteorder", R_OK): 1.3us
    - riscv_hwprobe() vDSO and syscall: .0094us
    - riscv_hwprobe() vDSO with no syscall: 0.0091us

    These numbers get farther apart if we query multiple keys, as sysfs will
    scale linearly with the number of keys, where the dedicated syscall
    stays the same. To frame these numbers, I also did a tight
    fork/exec/wait loop, which I measured as 4.8ms. So doing 4
    open/read/close operations is a delta of about 0.3%, versus a single vDSO
    call is a delta of essentially zero.

    [1] https://public-inbox.org/libc-alpha/20230206194819.1679472-1-evan@rivosinc.com/T/#t
    [2] https://pastebin.com/x84NEKaS

    Changes in v5:
    - Added tags
    - Fixed misuse of ISA_EXT_c as bitmap, changed to use
    riscv_isa_extension_available() (Heiko, Conor)
    - Document the alternatives approach in the commit message (Conor and
    Heiko).
    - Fix __init call warnings by making probe_vendor_features() and
    thead_feature_probe_func() __init_or_module.
    - Fixed compat vdso compilation failure (lkp).

    Changes in v4:
    - Used real types in syscall prototypes (Arnd)
    - Fixed static line break in do_riscv_hwprobe() (Conor)
    - Added newlines between documentation lists (Conor)
    - Crispen up size types to size_t, and cpu indices to int (Joe)
    - Fix copy_from_user() return logic bug (found via kselftests!)
    - Add __user to SYSCALL_DEFINE() to fix warning
    - More newlines in BASE_BEHAVIOR_IMA documentation (Conor)
    - Add newlines to CPUPERF_0 documentation (Conor)
    - Add UNSUPPORTED value (Conor)
    - Switched from DT to alternatives-based probing (Rob)
    - Crispen up cpu index type to always be int (Conor)
    - Fixed selftests commit description, no more tiny libc (Mark Brown)
    - Fixed selftest syscall prototype types to match v4.
    - Added a prototype to fix -Wmissing-prototype warning (lkp@intel.com)
    - Fixed rv32 build failure (lkp@intel.com)
    - Make vdso prototype match syscall types update

    Changes in v3:
    - Updated copyright date in cpufeature.h
    - Fixed typo in cpufeature.h comment (Conor)
    - Refactored functions so that kernel mode can query too, in
    preparation for the vDSO data population.
    - Changed the vendor/arch/imp IDs to return a value of -1 on mismatch
    rather than failing the whole call.
    - Const cpumask pointer in hwprobe_mid()
    - Embellished documentation WRT cpu_set and the returned values.
    - Renamed hwprobe_mid() to hwprobe_arch_id() (Conor)
    - Fixed machine ID doc warnings, changed elements to c:macro:.
    - Completed dangling unistd.h comment (Conor)
    - Fixed line breaks and minor logic optimization (Conor).
    - Use riscv_cached_mxxxid() (Conor)
    - Refactored base ISA behavior probe to allow kernel probing as well,
    in prep for vDSO data initialization.
    - Fixed doc warnings in IMA text list, use :c:macro:.
    - Have hwprobe_misaligned return int instead of long.
    - Constify cpumask pointer in hwprobe_misaligned()
    - Fix warnings in _PERF_O list documentation, use :c:macro:.
    - Move include cpufeature.h to misaligned patch.
    - Fix documentation mismatch for RISCV_HWPROBE_KEY_CPUPERF_0 (Conor)
    - Use for_each_possible_cpu() instead of NR_CPUS (Conor)
    - Break early in misaligned access iteration (Conor)
    - Increase MISALIGNED_MASK from 2 bits to 3 for possible UNSUPPORTED future
    value (Conor)
    - Introduced vDSO function

    Changes in v2:
    - Factored the move of struct riscv_cpuinfo to its own header
    - Changed the interface to look more like poll(). Rather than supplying
    key_offset and getting back an array of values with numerically
    contiguous keys, have the user pre-fill the key members of the array,
    and the kernel will fill in the corresponding values. For any key it
    doesn't recognize, it will set the key of that element to -1. This
    allows usermode to quickly ask for exactly the elements it cares
    about, and not get bogged down in a back and forth about newer keys
    that older kernels might not recognize. In other words, the kernel
    can communicate that it doesn't recognize some of the keys while
    still providing the data for the keys it does know.
    - Added a shortcut to the cpuset parameters that if a size of 0 and
    NULL is provided for the CPU set, the kernel will use a cpu mask of
    all online CPUs. This is convenient because I suspect most callers
    will only want to act on a feature if it's supported on all CPUs, and
    it's a headache to dynamically allocate an array of all 1s, not to
    mention a waste to have the kernel loop over all of the offline bits.
    - Fixed logic error in if(of_property_read_string...) that caused crash
    - Include cpufeature.h in cpufeature.h to avoid undeclared variable
    warning.
    - Added a _MASK define
    - Fix random checkpatch complaints
    - Updated the selftests to the new API and added some more.
    - Fixed indentation, comments in .S, and general checkpatch complaints.

    Evan Green (6):
    RISC-V: Move struct riscv_cpuinfo to new header
    RISC-V: Add a syscall for HW probing
    RISC-V: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR_IMA
    RISC-V: hwprobe: Support probing of misaligned access performance
    selftests: Test the new RISC-V hwprobe interface
    RISC-V: Add hwprobe vDSO function and data

    Documentation/riscv/hwprobe.rst | 86 +++++++
    Documentation/riscv/index.rst | 1 +
    arch/riscv/Kconfig | 1 +
    arch/riscv/errata/thead/errata.c | 10 +
    arch/riscv/include/asm/alternative.h | 5 +
    arch/riscv/include/asm/cpufeature.h | 23 ++
    arch/riscv/include/asm/hwprobe.h | 13 +
    arch/riscv/include/asm/syscall.h | 4 +
    arch/riscv/include/asm/vdso/data.h | 17 ++
    arch/riscv/include/asm/vdso/gettimeofday.h | 8 +
    arch/riscv/include/uapi/asm/hwprobe.h | 37 +++
    arch/riscv/include/uapi/asm/unistd.h | 9 +
    arch/riscv/kernel/alternative.c | 19 ++
    arch/riscv/kernel/compat_vdso/Makefile | 2 +-
    arch/riscv/kernel/cpu.c | 8 +-
    arch/riscv/kernel/cpufeature.c | 3 +
    arch/riscv/kernel/smpboot.c | 1 +
    arch/riscv/kernel/sys_riscv.c | 225 +++++++++++++++++-
    arch/riscv/kernel/vdso.c | 6 -
    arch/riscv/kernel/vdso/Makefile | 4 +
    arch/riscv/kernel/vdso/hwprobe.c | 52 ++++
    arch/riscv/kernel/vdso/sys_hwprobe.S | 15 ++
    arch/riscv/kernel/vdso/vdso.lds.S | 3 +
    tools/testing/selftests/Makefile | 1 +
    tools/testing/selftests/riscv/Makefile | 58 +++++
    .../testing/selftests/riscv/hwprobe/Makefile | 10 +
    .../testing/selftests/riscv/hwprobe/hwprobe.c | 90 +++++++
    .../selftests/riscv/hwprobe/sys_hwprobe.S | 12 +
    28 files changed, 709 insertions(+), 14 deletions(-)
    create mode 100644 Documentation/riscv/hwprobe.rst
    create mode 100644 arch/riscv/include/asm/cpufeature.h
    create mode 100644 arch/riscv/include/asm/hwprobe.h
    create mode 100644 arch/riscv/include/asm/vdso/data.h
    create mode 100644 arch/riscv/include/uapi/asm/hwprobe.h
    create mode 100644 arch/riscv/kernel/vdso/hwprobe.c
    create mode 100644 arch/riscv/kernel/vdso/sys_hwprobe.S
    create mode 100644 tools/testing/selftests/riscv/Makefile
    create mode 100644 tools/testing/selftests/riscv/hwprobe/Makefile
    create mode 100644 tools/testing/selftests/riscv/hwprobe/hwprobe.c
    create mode 100644 tools/testing/selftests/riscv/hwprobe/sys_hwprobe.S

    --
    2.25.1

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