linux-kernel - Re: [PATCH v2] arm64/sve: Document that __SVE_VQ

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Message-ID: <ZcpNV9wws5poOY8w@e133344.arm.com>
Date: Mon, 12 Feb 2024 16:54:47 +0000
From: Dave Martin <Dave.Martin@....com>
To: Mark Brown <broonie@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64/sve: Document that __SVE_VQ_MAX is much larger
 than needed

On Fri, Feb 09, 2024 at 04:53:36PM +0000, Mark Brown wrote:
> __SVE_VQ_MAX is defined without comment as 512 but the actual
> architectural maximum is 16, a substantial difference which might not
> be obvious to readers especially given the several different units used
> for specifying vector sizes in various contexts and the fact that it's
> often used via macros.  In an effort to minimise surprises for users who
> might assume the value is the architectural maximum and use it to do
> things like size allocations add a comment noting the difference, and
> add a note for SVE_VQ_MAX to aid discoverability.
> 
> Signed-off-by: Mark Brown <broonie@...nel.org>
> ---
> Changes in v2:
> - Use new wording mostly provided by Dave Martin.
> - Reference __SVE_VQ_MAX comment near SVE_VQ_MAX define.
> - Link to v1: https://lore.kernel.org/r/20240206-arm64-sve-vl-max-comment-v1-1-dddf16414412@kernel.org
> ---
>  arch/arm64/include/uapi/asm/sve_context.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/include/uapi/asm/sve_context.h b/arch/arm64/include/uapi/asm/sve_context.h
> index 754ab751b523..72aefc081061 100644
> --- a/arch/arm64/include/uapi/asm/sve_context.h
> +++ b/arch/arm64/include/uapi/asm/sve_context.h
> @@ -13,6 +13,17 @@
>  
>  #define __SVE_VQ_BYTES		16	/* number of bytes per quadword */
>  
> +/*
> + * Yes, __SVE_VQ_MAX is 512 QUADWORDS.
> + *
> + * To help ensure forward portability, this is much larger than the
> + * current maximum value defined by the SVE architecture.  While arrays
> + * or static allocations can be sized based on this value, watch out!
> + * It will waste a surprisingly large amount of memory.
> + *
> + * Dynamic sizing based on the actual runtime vector length is likely to
> + * be preferable for most purposes.
> + */
>  #define __SVE_VQ_MIN		1
>  #define __SVE_VQ_MAX		512

Acked-by: Dave Martin <Dave.Martin@....com>

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