linux-kernel - Re: [PATCH] arm64: cpufeature: Export Armv8.6 Matrix feature to userspace

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Date:   Tue, 29 Oct 2019 11:15:17 +0000
From:   Will Deacon <will@...nel.org>
To:     Julien Grall <julien.grall@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, suzuki.poulose@....com,
        Dave.Martin@....com
Subject: Re: [PATCH] arm64: cpufeature: Export Armv8.6 Matrix feature to
 userspace

On Fri, Oct 25, 2019 at 06:10:56PM +0100, Julien Grall wrote:
> This patch provides support for reporting the presence of Armv8.6
> Matrix and its optional features to userspace.

Are you sure this is 8.6 and not earlier?

> This based on [1] + commit ec52c7134b1f "arm64: cpufeature: Treat
> ID_AA64ZFR0_EL1 as RAZ when SVE is not enabled" (taken from v5.4-rc4).
> 
> [1]  arm64/for-next/elf-hwcap-docs
> ---
>  Documentation/arm64/cpu-feature-registers.rst |  8 ++++++++
>  Documentation/arm64/elf_hwcaps.rst            | 15 +++++++++++++++
>  arch/arm64/include/asm/hwcap.h                |  4 ++++
>  arch/arm64/include/asm/sysreg.h               |  7 +++++++
>  arch/arm64/include/uapi/asm/hwcap.h           |  4 ++++
>  arch/arm64/kernel/cpufeature.c                | 11 +++++++++++
>  arch/arm64/kernel/cpuinfo.c                   |  4 ++++
>  7 files changed, 53 insertions(+)
> 
> diff --git a/Documentation/arm64/cpu-feature-registers.rst b/Documentation/arm64/cpu-feature-registers.rst
> index ffcf4e2c71ef..d1d6d56a7b08 100644
> --- a/Documentation/arm64/cpu-feature-registers.rst
> +++ b/Documentation/arm64/cpu-feature-registers.rst
> @@ -193,6 +193,8 @@ infrastructure:
>       +------------------------------+---------+---------+
>       | Name                         |  bits   | visible |
>       +------------------------------+---------+---------+
> +     | I8MM                         | [52-55] |    y    |
> +     +------------------------------+---------+---------+

Looking at:

https://developer.arm.com/docs/ddi0601/latest/aarch64-system-registers/id_aa64isar1_el1

Then I8MM is advertised as "Armv8.2", alongside other fields that we haven't
listed here such as BF16 and SPECRES.

So we probably want a patch bringing all of this up to speed, rather than
randomly advertising some features and not others.

>       | SB                           | [36-39] |    y    |
>       +------------------------------+---------+---------+
>       | FRINTTS                      | [32-35] |    y    |
> @@ -227,6 +229,12 @@ infrastructure:
>       +------------------------------+---------+---------+
>       | Name                         |  bits   | visible |
>       +------------------------------+---------+---------+
> +     | F64MM                        | [56-59] |    y    |
> +     +------------------------------+---------+---------+
> +     | F32MM                        | [52-55] |    y    |
> +     +------------------------------+---------+---------+
> +     | I8MM                         | [44-47] |    y    |
> +     +------------------------------+---------+---------+

Urgh, we're inconsistent in our bitfields. Some are [lo-hi] whilst others
are [hi-lo]. Please can you fix that in a preparatory patch? I prefer
[hi-lo] and it matches the arch docs.

Will