linux-kernel - Re: [PATCH v2 3/6] RISC-V: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR

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Message-ID: <Y+1NvsLlbo8HvV5w@spud>
Date:   Wed, 15 Feb 2023 21:25:18 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Evan Green <evan@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...osinc.com>, vineetg@...osinc.com,
        heiko@...ech.de, slewis@...osinc.com,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrew Bresticker <abrestic@...osinc.com>,
        Celeste Liu <coelacanthus@...look.com>,
        Guo Ren <guoren@...nel.org>, Jonathan Corbet <corbet@....net>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        dram <dramforever@...e.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v2 3/6] RISC-V: hwprobe: Add support for
 RISCV_HWPROBE_BASE_BEHAVIOR_IMA

On Mon, Feb 06, 2023 at 12:14:52PM -0800, Evan Green wrote:
> From: Palmer Dabbelt <palmer@...osinc.com>
> 
> We have an implicit set of base behaviors that userspace depends on,
> which are mostly defined in various ISA specifications.
> 
> Signed-off-by: Palmer Dabbelt <palmer@...osinc.com>
> Signed-off-by: Evan Green <evan@...osinc.com>
> ---
> 
> (no changes since v1)
> 
>  Documentation/riscv/hwprobe.rst       | 16 ++++++++++++++++
>  arch/riscv/include/asm/hwprobe.h      |  2 +-
>  arch/riscv/include/uapi/asm/hwprobe.h |  6 +++++-
>  arch/riscv/kernel/sys_riscv.c         | 23 +++++++++++++++++++++++
>  4 files changed, 45 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
> index 97771090e972..ce186967861f 100644
> --- a/Documentation/riscv/hwprobe.rst
> +++ b/Documentation/riscv/hwprobe.rst
> @@ -35,3 +35,19 @@ The following keys are defined:
>    specifications.
>  * :RISCV_HWPROBE_KEY_MIMPLID:: Contains the value of :mimplid:, as per the ISA
>    specifications.
> +* :RISCV_HWPROBE_KEY_BASE_BEHAVIOR:: A bitmask containing the base user-visible
> +  behavior that this kernel supports.  The following base user ABIs are defined:
> +    * :RISCV_HWPROBE_BASE_BEHAVIOR_IMA:: Support for rv32ima or rv64ima, as
> +      defined by version 2.2 of the user ISA and version 1.10 of the privileged
> +      ISA, with the following known exceptions (more exceptions may be added,
> +      but only if it can be demonstrated that the user ABI is not broken):
> +        * The :fence.i: instruction cannot be directly executed by userspace
> +          programs (it may still be executed in userspace via a
> +          kernel-controlled mechanism such as the vDSO).

I don't really do the whole rst thing at all, are we able to have
newlines between list items? If we can, I think one would go nicely here.

> +* :RISCV_HWPROBE_KEY_IMA_EXT_0:: A bitmask containing the extensions that are
> +  compatible with the :RISCV_HWPROBE_BASE_BEHAVIOR_IMA: base system behavior.

Why do we specifically care if they're compatible with IMA?
What's the "fear" here?

> +    * :RISCV_HWPROBE_IMA_FD:: The F and D extensions are supported, as defined

Also, is this IMA and FD thing a kinda commitment to only supporting
hardware that has IMA* or IMAFD*
I know that's what we do now, but only under the hood?
As per usual, I'm probably missing something. What is it?

> +      by commit cd20cee ("FMIN/FMAX now implement minimumNumber/maximumNumber,
> +      not minNum/maxNum") of the RISC-V ISA manual.
> +    * :RISCV_HWPROBE_IMA_C:: The C extension is supported, as defined by
> +      version 2.2 of the RISC-V ISA manual.

See, this seems to be how we have to treat specs, list the exact
versions! I don't even have to look to know that this was in the v1 ;)


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