linux-kernel - Re: [PATCH] arm64: ARM Scalable Matrix Extension is ARMv9-A

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Message-ID: <20220510091502.GA27219@willie-the-truck>
Date:   Tue, 10 May 2022 10:15:02 +0100
From:   Will Deacon <will@...nel.org>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: ARM Scalable Matrix Extension is ARMv9-A

On Mon, May 09, 2022 at 03:40:30PM +0200, Geert Uytterhoeven wrote:
> The help text for the config option to enable support for the ARM
> Scalable Matrix Extension does not mention the specification baseline
> for this extension.  Make it clear this is an extension for the ARMv9-A
> architecture, so users know when it is safe to disable this option,
> which is enabled by default.
> 
> Fixes: a1f4ccd25cc25625 ("arm64/sme: Provide Kconfig for SME")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> ---
>  arch/arm64/Kconfig | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 0e9bbeb1262d0ca8..f3a0d0cff3fb5265 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1964,9 +1964,9 @@ config ARM64_SME
>  	default y
>  	depends on ARM64_SVE
>  	help
> -	  The Scalable Matrix Extension (SME) is an extension to the AArch64
> -	  execution state which utilises a substantial subset of the SVE
> -	  instruction set, together with the addition of new architectural
> +	  The Scalable Matrix Extension (SME) is an ARMv9-A extension to the
> +	  AArch64 execution state which utilises a substantial subset of the
> +	  SVE instruction set, together with the addition of new architectural

Why is this useful information? The v9 vs v8 distinction is purely a
marketing thing, so I'd be _very_ wary of drawing any technical conclusions
based on that, especially as Arm have a track record for "backporting"
features into older CPUs if there is demand for it. Do you know what the
toolchains have done here? (i.e. is there march=armv9-a or can you do
march=armv8-a+sme or something else?).

Having said all that, we do already have the Kconfig menu entries for "Armv8.x
architectural features", so adding one for "Armv9" would be consistent with
that if you really want it.

Will