linux-kernel - Re: [PATCH 3/4] arm64/fp: Clarify effect of setting an unsupported system VL

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Message-ID: <Za/gB3oCNAyiPPnu@e133380.arm.com>
Date: Tue, 23 Jan 2024 15:49:27 +0000
From: Dave Martin <Dave.Martin@....com>
To: Mark Brown <broonie@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Edmund Grimley-Evans <edmund.grimley-evans@....com>
Subject: Re: [PATCH 3/4] arm64/fp: Clarify effect of setting an unsupported
 system VL

On Mon, Jan 22, 2024 at 08:41:53PM +0000, Mark Brown wrote:
> The documentation for system vector length configuration does not cover all
> cases where unsupported values are written, tighten it up.
> 
> Reported-by: Edmund Grimley-Evans <edmund.grimley-evans@....com>
> Signed-off-by: Mark Brown <broonie@...nel.org>
> ---
>  Documentation/arch/arm64/sme.rst | 5 ++---
>  Documentation/arch/arm64/sve.rst | 5 ++---
>  2 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/arch/arm64/sme.rst b/Documentation/arch/arm64/sme.rst
> index 3133d0e91b48..ba0a5e5b2523 100644
> --- a/Documentation/arch/arm64/sme.rst
> +++ b/Documentation/arch/arm64/sme.rst
> @@ -379,9 +379,8 @@ The regset data starts with struct user_za_header, containing:
>  /proc/sys/abi/sme_default_vector_length
>  
>      Writing the text representation of an integer to this file sets the system
> -    default vector length to the specified value, unless the value is greater
> -    than the maximum vector length supported by the system in which case the
> -    default vector length is set to that maximum.
> +    default vector length to the specified value rounded to a supported value
> +    using the same rules as for setting vector length via prctl().

Do parallel changes need to be made in sve.rst?

(There seems to be so much duplication and copy-paste between these
files that I wonder whether it would make sense to merge them...  but
that's probably a separate discussion.)

Nit: is it better to name the prctl here than just to say prctl()?
That would be easier for the reader to cross-reference.

>  
>      The result can be determined by reopening the file and reading its
>      contents.
> diff --git a/Documentation/arch/arm64/sve.rst b/Documentation/arch/arm64/sve.rst
> index b45a2da19bf1..b923727ff4b9 100644
> --- a/Documentation/arch/arm64/sve.rst
> +++ b/Documentation/arch/arm64/sve.rst
> @@ -423,9 +423,8 @@ The regset data starts with struct user_sve_header, containing:
>  /proc/sys/abi/sve_default_vector_length
>  
>      Writing the text representation of an integer to this file sets the system
> -    default vector length to the specified value, unless the value is greater
> -    than the maximum vector length supported by the system in which case the
> -    default vector length is set to that maximum.
> +    default vector length to the specified value rounded to a supported value
> +    using the same rules as for setting vector length via prctl().

Ditto.

[...]

Cheers
---Dave