Re: [PATCH v3 8/8] RISC-V: hwprobe: Document unaligned vector perf key

[Evan Green <evan@rivosinc.com>]

On Mon, Jun 24, 2024 at 5:52 PM Jesse Taube <jesse@rivosinc.com> wrote:
>
> Document key for reporting the speed of unaligned vector accesses.
> The descriptions are the same as the scalar equivalent values.
>
> Signed-off-by: Jesse Taube <jesse@rivosinc.com>
> ---
> V1 -> V2:
>   - New patch
> V2 -> V3:
>  - Specify access width
> ---
>  Documentation/arch/riscv/hwprobe.rst | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
> index 7085a694b801..d102b4a16d55 100644
> --- a/Documentation/arch/riscv/hwprobe.rst
> +++ b/Documentation/arch/riscv/hwprobe.rst
> @@ -236,3 +236,19 @@ The following keys are defined:
>
>  * :c:macro:`RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE`: An unsigned int which
>    represents the size of the Zicboz block in bytes.
> +
> +* :c:macro:`RISCV_HWPROBE_KEY_VEC_MISALIGNED_PERF`: An enum value describing the
> +  performance of misaligned vector accesses on the selected set of processors.
> +
> +  * :c:macro:`RISCV_HWPROBE_VEC_MISALIGNED_UNKNOWN`: The performance of misaligned
> +    accesses is unknown.
> +
> +  * :c:macro:`RISCV_HWPROBE_VEC_MISALIGNED_SLOW`: 32bit misaligned accesses are slower
> +    than equivalent byte accesses.  Misaligned accesses may be supported

Do you think it's worth specifying that we're talking about byte
accesses using vector registers? In other words, clarifying that we're
not comparing misaligned vector loads to loads/stores into the scalar
registers. Maybe something like:

32-bit misaligned accesses using vector registers are slower than the
equivalent quantity of byte accesses via vector registers. Misaligned
accesses may ...

-Evan

> +    directly in hardware, or trapped and emulated by software.
> +
> +  * :c:macro:`RISCV_HWPROBE_VEC_MISALIGNED_FAST`: 32bit misaligned accesses are faster
> +    than equivalent byte accesses.
> +
> +  * :c:macro:`RISCV_HWPROBE_VEC_MISALIGNED_UNSUPPORTED`: Misaligned accesses are
> +    not supported at all and will generate a misaligned address fault.
> --
> 2.45.2
>