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Date:   Thu, 28 Apr 2022 10:19:22 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Mark Brown <broonie@kernel.org>
Cc:     Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Shuah Khan <shuah@kernel.org>,
        Alan Hayward <alan.hayward@arm.com>,
        Luis Machado <luis.machado@arm.com>,
        Salil Akerkar <Salil.Akerkar@arm.com>,
        Basant Kumar Dwivedi <Basant.KumarDwivedi@arm.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Luca Salabrino <luca.scalabrino@arm.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-kselftest@vger.kernel.org, kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH v14 04/39] arm64/sme: Provide ABI documentation for SME
Message-ID: <YmpcGpSYpoiprngy@arm.com>
References: <20220419112247.711548-1-broonie@kernel.org>
 <20220419112247.711548-5-broonie@kernel.org>
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On Tue, Apr 19, 2022 at 12:22:12PM +0100, Mark Brown wrote:
> +* There are a number of optional SME features, presence of these is reported
> +  through AT_HWCAP2 through:
> +
> +	HWCAP2_SME_I16I64
> +	HWCAP2_SME_F64F64
> +	HWCAP2_SME_I8I32
> +	HWCAP2_SME_F16F32
> +	HWCAP2_SME_B16F32
> +	HWCAP2_SME_F32F32
> +	HWCAP2_SME_FA64

Marc pointed out that in combination with FEAT_WFxT, we used all the
HWCAP2 bits (32). While we are ok for now, we'll soon need to look into
what to do when the next features turn up. Some options:

1. Only provide HWCAP2_SME and let the ID_AA64SMFR0_EL1 features be
   probed via MRS emulation. It doesn't solve the problem but it buys us
   a bit of time.

2. Don't bother with any new HWCAPs, just rely on MRS emulation (we have
   HWCAP_CPUID advertising this).

3. Start using the upper 32-bit of HWCAP and HWCAP2 (we initially didn't
   go into these as there was a slight chance of merging ILP32). Does
   the libc rely on the upper bits for anything? Or does it just assume
   a 32-bit HWCAPs layout?

4. Introduce HWCAP3.

Szabolcs, any thoughts?

Thanks.

-- 
Catalin