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Re: [PATCH v2 3/3] target/arm: Add sve-default-vector-length cpu propert

From: Peter Maydell
Subject: Re: [PATCH v2 3/3] target/arm: Add sve-default-vector-length cpu property
Date: Mon, 26 Jul 2021 12:07:09 +0100

On Fri, 23 Jul 2021 at 21:36, Richard Henderson
<richard.henderson@linaro.org> wrote:
> Mirror the behavour of /proc/sys/abi/sve_default_vector_length
> under the real linux kernel.  We have no way of passing along
> a real default across exec like the kernel can, but this is a
> decent way of adjusting the startup vector length of a process.
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/482
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  docs/system/arm/cpu-features.rst | 11 ++++++
>  target/arm/cpu.h                 |  5 +++
>  target/arm/cpu.c                 | 14 ++++++--
>  target/arm/cpu64.c               | 60 ++++++++++++++++++++++++++++++++
>  4 files changed, 88 insertions(+), 2 deletions(-)
> diff --git a/docs/system/arm/cpu-features.rst 
> b/docs/system/arm/cpu-features.rst
> index c455442eaf..4ff36cc83f 100644
> --- a/docs/system/arm/cpu-features.rst
> +++ b/docs/system/arm/cpu-features.rst
> @@ -376,3 +376,14 @@ verbose command lines.  However, the recommended way to 
> select vector
>  lengths is to explicitly enable each desired length.  Therefore only
>  example's (1), (4), and (6) exhibit recommended uses of the properties.
> +SVE User-mode Default Vector Length Property
> +--------------------------------------------
> +
> +For qemu-aarch64, the cpu property `sve-default-vector-length=N` is

You probably don't want single-backticks. In rST this means
"interpreted text", which can be handled as a bunch of different
things if tagged with a specific "role":
The default "role" if none is specified is "title_reference", intended for
references to book or article titles, and it renders into the HTML
as <cite>...</cite> (usually comes out as italics).

If you want fixed-width-font text, that's double backtick: ``text``.
If you want italics, that's *emphasised text*.
If you want bold, that's **strong text**.

> +defined to mirror the Linux kernel parameter file
> +`/proc/sys/abi/sve_default_vector_length`.  The default length, `N`,
> +is in units of bytes and must be between 16 and 8192.
> +If not specified, the default vector length is 64.
> +
> +If the default length is larger than the maximum vector length enabled
> +with `sve<N>` properties, the actual vector length will be reduced.

We should document the -1 case too, something like:

If this property is set to ``-1`` then the default vector length
is set to the maximum possible length.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

-- PMM

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