linux-kernel - Re: [PATCH] arm64/sme: Add hwcap for Scalable Matrix Extension

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Message-ID: <YlgNW0/ji6KlkyZo@sirena.org.uk>
Date:   Thu, 14 Apr 2022 13:02:35 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jonathan Corbet <corbet@....net>,
        Marc Zyngier <maz@...nel.org>, Joey Gouly <joey.gouly@....com>,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Fuad Tabba <tabba@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/sme: Add hwcap for Scalable Matrix Extension

On Thu, Apr 14, 2022 at 07:55:44PM +0800, Tianjia Zhang wrote:

> Allow userspace to detect support for SME (Scalable Matrix Extension)
> by providing a hwcap for it, using the official feature name FEAT_SME,
> declared in ARM DDI 0487H.a specification.

There's already a hwcap for the core feature and all the subfeatures
added as part of the series I've been posting for SME:

   https://lore.kernel.org/linux-arm-kernel/20220408114328.1401034-1-broonie@kernel.org/

Why add something independently, especially given that there is no way
for userspace to do anything constructive with the feature without the
rest of the kernel support?  Any attempt to use SME instructions without
kernel support will trap and generate a SIGILL even if the feature is
present in hardware.

Do you have a system with SME that you're trying to use?  Review/testing
on the current series would be appreciated.

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