Re: [PATCH v2 0/6] RISC-V Hardware Probing User Interface

[Conor Dooley]

Hey Evan,
Having been talking to Palmer about this series at FOSDEM,
it was a very pleasant surprise to see this when I saw this in my inbox when I landed back home.
I do very much intend reviewing this, but... 

On 6 February 2023 20:14:49 GMT, Evan Green <evan@xxxxxxxxxxxx> wrote:
>
>These are very much up for discussion, as it's a pretty big new user
>interface and it's quite a bit different from how we've historically
>done things: this isn't just providing an ISA string to userspace, this
>has its own format for providing information to userspace.
>
>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.  I'm not really sure that's the
>right way to go about this, but it makes for flexible prototying.
>Various other approaches have been talked about like making HWCAP2 a
>pointer, having a VDSO routine, or exposing this via sysfs.  Those seem
>like generally reasonable approaches, but I've yet to figure out a way

This all looks to be the same cover message as Palmer submitted with the v1,
so, as I'd mentioned to him the other day, I'd like to do a bit
of an investigation into the sysfs approach drew suggested
on the v1.
So, if it's a little bit before you hear - I've certainly not forgotten about the series!

Thanks,
Conor.

>to get the general case working without a syscall as that's the only way
>I've come up with to deal with the heterogenous CPU case.  Happy to hear
>if someone has a better idea, though, as I don't really want to add a
>syscall if we can avoid it.
>
>An example series in glibc exposing this syscall and using it in an
>ifunc selector for memcpy can be found at [1].
>
>[1] https://public-inbox.org/libc-alpha/20230206194819.1679472-1-evan@xxxxxxxxxxxx/T/#t
>
>Changes in v2:
> - 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 (4):
>  RISC-V: Move struct riscv_cpuinfo to new header
>  RISC-V: Add a syscall for HW probing
>  RISC-V: hwprobe: Support probing of misaligned access performance
>  selftests: Test the new RISC-V hwprobe interface
>
>Palmer Dabbelt (2):
>  RISC-V: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR_IMA
>  dt-bindings: Add RISC-V misaligned access performance
>
> .../devicetree/bindings/riscv/cpus.yaml       |  15 ++
> Documentation/riscv/hwprobe.rst               |  66 ++++++
> Documentation/riscv/index.rst                 |   1 +
> arch/riscv/include/asm/cpufeature.h           |  23 +++
> arch/riscv/include/asm/hwprobe.h              |  13 ++
> arch/riscv/include/asm/smp.h                  |   9 +
> arch/riscv/include/asm/syscall.h              |   3 +
> arch/riscv/include/uapi/asm/hwprobe.h         |  35 ++++
> arch/riscv/include/uapi/asm/unistd.h          |   8 +
> arch/riscv/kernel/cpu.c                       |  11 +-
> arch/riscv/kernel/cpufeature.c                |  31 ++-
> arch/riscv/kernel/sys_riscv.c                 | 192 +++++++++++++++++-
> tools/testing/selftests/Makefile              |   1 +
> tools/testing/selftests/riscv/Makefile        |  58 ++++++
> .../testing/selftests/riscv/hwprobe/Makefile  |  10 +
> .../testing/selftests/riscv/hwprobe/hwprobe.c |  89 ++++++++
> .../selftests/riscv/hwprobe/sys_hwprobe.S     |  12 ++
> tools/testing/selftests/riscv/libc.S          |  46 +++++
> 18 files changed, 613 insertions(+), 10 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/uapi/asm/hwprobe.h
> 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
> create mode 100644 tools/testing/selftests/riscv/libc.S
>