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Date:   Sun, 24 Apr 2022 17:27:09 +0800
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Subject: Re: [PATCH] arm64/sme: Add hwcap for Scalable Matrix Extension
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To:     Mark Brown <broonie@kernel.org>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Marc Zyngier <maz@kernel.org>, Joey Gouly <joey.gouly@arm.com>,
        Vincenzo Frascino <Vincenzo.Frascino@arm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Fuad Tabba <tabba@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Vladimir Murzin <vladimir.murzin@arm.com>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20220414115544.36204-1-tianjia.zhang@linux.alibaba.com>
 <YlgNW0/ji6KlkyZo@sirena.org.uk>
 <e7a0d0d4-b6ca-6fec-df33-929961f0d43e@linux.alibaba.com>
 <Yl6/+baDhElVxwyC@sirena.org.uk>
From:   Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
In-Reply-To: <Yl6/+baDhElVxwyC@sirena.org.uk>
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Hi Mark,

On 4/19/22 9:58 PM, Mark Brown wrote:
> On Fri, Apr 15, 2022 at 10:25:33AM +0800, Tianjia Zhang wrote:
>> On 4/14/22 8:02 PM, Mark Brown wrote:
>>> On Thu, Apr 14, 2022 at 07:55:44PM +0800, Tianjia Zhang wrote:
> 
>>> 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.
> 
> I see.  Unfortunately all the new registers mean that we really need to
> define all the ABI as soon as we enable anything and the only thing we
> can really skip out on when doing initial enablement is KVM (which I
> have in fact skipped for the time being, I'll look at that at some point
> after the initial support is landed).
> 
>> 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.
> 
> I'm not aware of anything, but I am mostly focused on the OS support
> rather than any of the actual mathematical operations that are more the
> point of these architecture features so I might be missing something.
> 
>>> 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?
> 
> Other than verifying that the series doesn't cause trouble for systems
> without SME

Thanks for your reply, I have indirectly implemented the functionality
of the REVD instruction using the tbl instruction on a machine that does
not support SME.

For this group of patchsets, I will do some tests later, which may take
a long time, and there is currently no exclusive machine at hand.

Best regards,
Tianjia