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Date:   Wed, 18 Oct 2023 18:36:59 +0100
From:   Conor Dooley <conor@kernel.org>
To:     Evan Green <evan@rivosinc.com>
Cc:     =?iso-8859-1?Q?Cl=E9ment_L=E9ger?= <cleger@rivosinc.com>,
        linux-riscv@lists.infradead.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        Palmer Dabbelt <palmer@rivosinc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Jones <ajones@ventanamicro.com>,
        Samuel Ortiz <sameo@rivosinc.com>
Subject: Re: [PATCH v2 01/19] riscv: hwprobe: factorize hwprobe ISA extension
 reporting
Message-ID: <20231018-flagpole-footpad-07a6228485f3@spud>
References: <20231017131456.2053396-1-cleger@rivosinc.com>
 <20231017131456.2053396-2-cleger@rivosinc.com>
 <CALs-HssL=wNwj9nRuZwpZhy1CB9p9-X=OqgwBw9zvgA7hA4fEg@mail.gmail.com>
 <20231018-scrap-bankable-a0f321d97a46@spud>
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On Wed, Oct 18, 2023 at 06:33:34PM +0100, Conor Dooley wrote:
> On Wed, Oct 18, 2023 at 10:24:15AM -0700, Evan Green wrote:
> > On Tue, Oct 17, 2023 at 6:15=E2=80=AFAM Cl=C3=A9ment L=C3=A9ger <cleger=
@rivosinc.com> wrote:
> > >
> > > Factorize ISA extension reporting by using a macro rather than
> > > copy/pasting extension names. This will allow adding new extensions m=
ore
> > > easily.
> > >
> > > Signed-off-by: Cl=C3=A9ment L=C3=A9ger <cleger@rivosinc.com>
> > > ---
> > >  arch/riscv/kernel/sys_riscv.c | 32 ++++++++++++++++++--------------
> > >  1 file changed, 18 insertions(+), 14 deletions(-)
> > >
> > > diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_ri=
scv.c
> > > index 473159b5f303..e207874e686e 100644
> > > --- a/arch/riscv/kernel/sys_riscv.c
> > > +++ b/arch/riscv/kernel/sys_riscv.c
> > > @@ -145,20 +145,24 @@ static void hwprobe_isa_ext0(struct riscv_hwpro=
be *pair,
> > >         for_each_cpu(cpu, cpus) {
> > >                 struct riscv_isainfo *isainfo =3D &hart_isa[cpu];
> > >
> > > -               if (riscv_isa_extension_available(isainfo->isa, ZBA))
> > > -                       pair->value |=3D RISCV_HWPROBE_EXT_ZBA;
> > > -               else
> > > -                       missing |=3D RISCV_HWPROBE_EXT_ZBA;
> > > -
> > > -               if (riscv_isa_extension_available(isainfo->isa, ZBB))
> > > -                       pair->value |=3D RISCV_HWPROBE_EXT_ZBB;
> > > -               else
> > > -                       missing |=3D RISCV_HWPROBE_EXT_ZBB;
> > > -
> > > -               if (riscv_isa_extension_available(isainfo->isa, ZBS))
> > > -                       pair->value |=3D RISCV_HWPROBE_EXT_ZBS;
> > > -               else
> > > -                       missing |=3D RISCV_HWPROBE_EXT_ZBS;
> > > +#define CHECK_ISA_EXT(__ext)                                        =
           \
> > > +               do {                                                 =
           \
> > > +                       if (riscv_isa_extension_available(isainfo->is=
a, __ext)) \
> > > +                               pair->value |=3D RISCV_HWPROBE_EXT_##=
__ext;       \
> > > +                       else                                         =
           \
> > > +                               missing |=3D RISCV_HWPROBE_EXT_##__ex=
t;           \
> > > +               } while (false)
> > > +
> > > +               /*
> > > +                * Only use CHECK_ISA_EXT() for extensions which can =
be exposed
> > > +                * to userspace, regardless of the kernel's configura=
tion, as no
> > > +                * other checks, besides presence in the hart_isa bit=
map, are
> > > +                * made.
> >=20
> > This comment alludes to a dangerous trap, but I'm having trouble
> > understanding what it is.
>=20
> You cannot, for example, use this for communicating the presence of F or
> D, since they require a config option to be set before their use is
> safe.

Funnily enough, this comment is immediately contradicted by the vector
subset extensions, where these CHECK_ISA_EXT() macros are used wrapped
in has_vector(). The code looks valid to me, since has_vector() contains
the Kconfig check, but does fly in the face of this comment.

>=20
> > Perhaps some rewording to more explicitly
> > state the danger would be appropriate. Other than that:
> >=20
> > Reviewed-by: Evan Green <evan@rivosinc.com>


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