linux-kernel - Re: [PATCH v1 3/3] Documentation: riscv: add a section about ISA string ordering in /proc/cpuinfo

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Date:   Wed, 30 Nov 2022 23:46:28 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org
Cc:     Conor Dooley <conor.dooley@...rochip.com>, ajones@...tanamicro.com,
        aou@...s.berkeley.edu, corbet@....net, guoren@...nel.org,
        heiko@...ech.de, paul.walmsley@...ive.com,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v1 3/3] Documentation: riscv: add a section about ISA
 string ordering in /proc/cpuinfo

On Wed, Nov 30, 2022 at 11:41:26PM +0000, Conor Dooley wrote:
> From: Conor Dooley <conor.dooley@...rochip.com>
> 
> The RISC-V specs are permissive in what they allow as the ISA string,
> but how we output this to userspace in /proc/cpuinfo is quasi uAPI.
> 
> Formalise this as part of the uAPI, by documenting the list of rules
> we use at this point in time.
> 
> Signed-off-by: Conor Dooley <conor.dooley@...rochip.com>
> ---
> I've not "tested" these docs. The NIPA-esque pwbot should go and
> test it AFAICT. If it doesn't, I'll go add that.
> ---
>  Documentation/riscv/uabi.rst | 42 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/Documentation/riscv/uabi.rst b/Documentation/riscv/uabi.rst
> index 21a82cfb6c4d..bc3c8ced644b 100644
> --- a/Documentation/riscv/uabi.rst
> +++ b/Documentation/riscv/uabi.rst
> @@ -3,4 +3,46 @@
>  RISC-V Linux User ABI
>  =====================
>  
> +Misaligned accesses
> +-------------------
> +
>  Misaligned accesses are supported in userspace, but they may perform poorly.
> +
> +ISA string ordering in /proc/cpuinfo
> +------------------------------------
> +
> +The canonical order of ISA extension names in the ISA string is defined in
> +chapter 27 of the unprivileged specification.
> +The specification uses vague wording, such as should, when it comes to
> +ordering, so for our purposes the following rules apply:
> +
> +#. Single-letter extensions come first, in "canonical order", so
> +   "IMAFDQLCBKJTPVH".
> +
> +#. All multi-letter extensions will be separated from other multi-letter
> +   extensions by an underscore.
> +
> +#. Additional standard extensions (starting with 'Z') will be sorted after
> +   single-letter extensions and before any higher-privileged extensions.
> +
> +#. The first letter following the 'Z' conventionally indicates the most
> +   closely related alphabetical extension category, IMAFDQLCBKJTPVH.
> +   If multiple 'Z' extensions are named, they should be ordered first by
> +   category, then alphabetically within a category.
> +
> +#. Standard supervisor-level extensions (starting with 'S') will be listed
> +   after standard unprivileged extensions.  If multiple
> +   supervisor-level extensions are listed, they will be ordered
> +   alphabetically.
> +
> +#. Standard machine-level extensions (starting with 'Zxm') will be listed
> +   after any lower-privileged, standard extensions.  If multiple
> +   machine-level extensions are listed, they will be ordered
> +   alphabetically.
> +
> +#. Non-standard extensions (starts with 'X') will be listed after all

Ehh, it's always the read *after* sending something that I notice the
inconsistency. This should be s/starts/starting/ for consistency.

> +   standard extensions.
> +
> +An example string following the order is:
> +   rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux
> +
> -- 
> 2.38.1
>