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

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Date:   Fri, 15 Apr 2022 10:25:33 +0800
From:   Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
To:     Mark Brown <broonie@...nel.org>
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

Hi Mark,

On 4/14/22 8:02 PM, Mark Brown wrote:
> 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.

Great job, I encountered the issue of invalid REVD (requires FEAT_SME)
instruction when developing SVE2 programs, so I plan to gradually
support SME in the kernel, thanks for your contribution, you can ignore
my patch.

In addition, I would like to ask a question, whether there is an
alternative SVE2 instruction for the REVD instruction that can complete
this operation, if the machine does not support SME.

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

Unfortunately, the value currently read by my machine ID_AA64PFR1_EL1
register is 0x121. It seems that the hardware does not support SME. Is
there any other help I can provide?

Kind regards,
Tianjia