Re: [PATCH v2 0/3] RISC-V: ifunced memcpy using new kernel hwprobe interface

[Palmer Dabbelt <palmer@rivosinc.com>]

On Tue, 21 Feb 2023 11:15:34 PST (-0800), Evan Green wrote:
>
> This series illustrates the use of a proposed Linux syscall that
> enumerates architectural information about the RISC-V cores the system
> is running on. In this series we expose a small wrapper function around
> the syscall. An ifunc selector for memcpy queries it to see if unaligned
> access is "fast" on this hardware. If it is, it selects a newly provided
> implementation of memcpy that doesn't work hard at aligning the src and
> destination buffers.
>
> This is somewhat of a proof of concept for the syscall itself, but I do
> find that in my goofy memcpy test [1], the unaligned memcpy performed at
> least as well as the generic C version. This is however on Qemu on an M1
> mac, so not a test of any real hardware (more a smoke test that the
> implementation isn't silly).

QEMU isn't a good enough benchmark to justify a new memcpy routine in 
glibc.  Evan has a D1, which does support misaligned access and runs 
some simple benchmarks faster.  There's also been some minor changes to 
the Linux side of things that warrant a v3 anyway, so he'll just post 
some benchmarks on HW along with that.

Aside from those comments,

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>

There's a lot more stuff to probe for, but I think we've got enough of a 
proof of concept for the hwprobe stuff that we can move forward with the 
core interface bits in Linux/glibc and then unleash the chaos...

Unless anyone else has comments?

> v3 of the Linux series can be found at [2].
>
> [1] https://pastebin.com/Nj8ixpkX
> [2] https://lore.kernel.org/lkml/20230221190858.3159617-1-evan@rivosinc.com/T/#t
>
> Changes in v2:
>  - hwprobe.h: Use __has_include and duplicate Linux content to make
>    compilation work when Linux headers are absent (Adhemerval)
>  - hwprobe.h: Put declaration under __USE_GNU (Adhemerval)
>  - Use INLINE_SYSCALL_CALL (Adhemerval)
>  - Update versions
>  - Update UNALIGNED_MASK to match kernel v3 series.
>  - Add vDSO interface
>  - Used _MASK instead of _FAST value itself.
>
> Evan Green (3):
>   riscv: Add Linux hwprobe syscall support
>   riscv: Add hwprobe vdso call support
>   riscv: Add and use alignment-ignorant memcpy
>
>  sysdeps/riscv/memcopy.h                       |  28 +++++
>  sysdeps/riscv/memcpy.c                        |  65 +++++++++++
>  sysdeps/riscv/memcpy_noalignment.S            | 103 ++++++++++++++++++
>  sysdeps/unix/sysv/linux/dl-vdso-setup.c       |  10 ++
>  sysdeps/unix/sysv/linux/dl-vdso-setup.h       |   3 +
>  sysdeps/unix/sysv/linux/riscv/Makefile        |   8 +-
>  sysdeps/unix/sysv/linux/riscv/Versions        |   3 +
>  sysdeps/unix/sysv/linux/riscv/hwprobe.c       |  36 ++++++
>  .../unix/sysv/linux/riscv/memcpy-generic.c    |  24 ++++
>  .../unix/sysv/linux/riscv/rv32/arch-syscall.h |   1 +
>  .../unix/sysv/linux/riscv/rv32/libc.abilist   |   1 +
>  .../unix/sysv/linux/riscv/rv64/arch-syscall.h |   1 +
>  .../unix/sysv/linux/riscv/rv64/libc.abilist   |   1 +
>  sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h   |  67 ++++++++++++
>  sysdeps/unix/sysv/linux/riscv/sysdep.h        |   1 +
>  sysdeps/unix/sysv/linux/syscall-names.list    |   1 +
>  16 files changed, 351 insertions(+), 2 deletions(-)
>  create mode 100644 sysdeps/riscv/memcopy.h
>  create mode 100644 sysdeps/riscv/memcpy.c
>  create mode 100644 sysdeps/riscv/memcpy_noalignment.S
>  create mode 100644 sysdeps/unix/sysv/linux/riscv/hwprobe.c
>  create mode 100644 sysdeps/unix/sysv/linux/riscv/memcpy-generic.c
>  create mode 100644 sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h

Thanks!