Re: [PATCH v4 3/6] RISC-V: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR_IMA

From: Evan Green <evan@rivosinc.com>
To: "Heiko Stübner" <heiko@sntech.de>
Cc: Palmer Dabbelt <palmer@rivosinc.com>,
	slewis@rivosinc.com, Conor Dooley <conor@kernel.org>,
	vineetg@rivosinc.com, Conor Dooley <conor.dooley@microchip.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Andrew Bresticker <abrestic@rivosinc.com>,
	Celeste Liu <coelacanthus@outlook.com>,
	Guo Ren <guoren@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-riscv@lists.infradead.org
Subject: Re: [PATCH v4 3/6] RISC-V: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR_IMA
Date: Wed, 22 Mar 2023 09:17:34 -0700	[thread overview]
Message-ID: <CALs-HsuCejsEtS65pH=SBp74V2gw2d2WmkL44muWr6+VSocO5w@mail.gmail.com> (raw)
In-Reply-To: <3351107.e9J7NaK4W3@diego>

On Tue, Mar 21, 2023 at 9:41 AM Heiko Stübner <heiko@sntech.de> wrote:
>
> Hi Evan,
>
> Am Dienstag, 14. März 2023, 19:32:17 CET schrieb Evan Green:
> > We have an implicit set of base behaviors that userspace depends on,
> > which are mostly defined in various ISA specifications.
> >
> > Co-developed-by: Palmer Dabbelt <palmer@rivosinc.com>
> > Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> > Signed-off-by: Evan Green <evan@rivosinc.com>
> > Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
> > ---
> >
> > Changes in v4:
> >  - More newlines in BASE_BEHAVIOR_IMA documentation (Conor)
> >
> > Changes in v3:
> >  - Refactored base ISA behavior probe to allow kernel probing as well,
> >    in prep for vDSO data initialization.
> >  - Fixed doc warnings in IMA text list, use :c:macro:.
> >
> >  Documentation/riscv/hwprobe.rst       | 24 ++++++++++++++++++++++++
> >  arch/riscv/include/asm/hwprobe.h      |  2 +-
> >  arch/riscv/include/uapi/asm/hwprobe.h |  5 +++++
> >  arch/riscv/kernel/sys_riscv.c         | 20 ++++++++++++++++++++
> >  4 files changed, 50 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
> > index 211828f706e3..945d44683c40 100644
> > --- a/Documentation/riscv/hwprobe.rst
> > +++ b/Documentation/riscv/hwprobe.rst
> > @@ -39,3 +39,27 @@ The following keys are defined:
> >
> >  * :c:macro:`RISCV_HWPROBE_KEY_MIMPLID`: Contains the value of ``mimplid``, as
> >    defined by the RISC-V privileged architecture specification.
> > +
> > +* :c:macro:`RISCV_HWPROBE_KEY_BASE_BEHAVIOR`: A bitmask containing the base
> > +  user-visible behavior that this kernel supports.  The following base user ABIs
> > +  are defined:
> > +
> > +  * :c:macro:`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).
> > +
> > +* :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0`: A bitmask containing the extensions
> > +  that are compatible with the :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`:
> > +  base system behavior.
> > +
> > +  * :c:macro:`RISCV_HWPROBE_IMA_FD`: The F and D extensions are supported, as
> > +    defined by commit cd20cee ("FMIN/FMAX now implement
> > +    minimumNumber/maximumNumber, not minNum/maxNum") of the RISC-V ISA manual.
> > +
> > +  * :c:macro:`RISCV_HWPROBE_IMA_C`: The C extension is supported, as defined
> > +    by version 2.2 of the RISC-V ISA manual.
>
> just wondering, is there a plan on how further extensions should be added this this?
> [as we have this big plethora of them :-) ]
>
> Aka things like Zbb and friends will probably also be relevant to userspace, so just
> fill up RISCV_HWPROBE_KEY_IMA_EXT_0 with more elements and once full switch to
> RISCV_HWPROBE_KEY_IMA_EXT_1 , RISCV_HWPROBE_KEY_IMA_EXT_2, etc?
>
> Or do we have some more elaborate sorting mechanism?

That sounds reasonable to me. I tried to think about a couple of
possible sorting patterns, but when I played them out mentally they
were only ever aesthetically pleasing with no technical benefit, and
possibly added technical debt.
-Evan