linux-kernel - Re: [PATCH v2 01/19] riscv: hwprobe: factorize hwprobe ISA extension reporting

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Date:   Wed, 18 Oct 2023 18:36:59 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Evan Green <evan@...osinc.com>
Cc:     Clément Léger <cleger@...osinc.com>,
        linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Palmer Dabbelt <palmer@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Jonathan Corbet <corbet@....net>,
        Andrew Jones <ajones@...tanamicro.com>,
        Samuel Ortiz <sameo@...osinc.com>
Subject: Re: [PATCH v2 01/19] riscv: hwprobe: factorize hwprobe ISA extension
 reporting

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 AM Clément Léger <cleger@...osinc.com> wrote:
> > >
> > > Factorize ISA extension reporting by using a macro rather than
> > > copy/pasting extension names. This will allow adding new extensions more
> > > easily.
> > >
> > > Signed-off-by: Clément Léger <cleger@...osinc.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_riscv.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_hwprobe *pair,
> > >         for_each_cpu(cpu, cpus) {
> > >                 struct riscv_isainfo *isainfo = &hart_isa[cpu];
> > >
> > > -               if (riscv_isa_extension_available(isainfo->isa, ZBA))
> > > -                       pair->value |= RISCV_HWPROBE_EXT_ZBA;
> > > -               else
> > > -                       missing |= RISCV_HWPROBE_EXT_ZBA;
> > > -
> > > -               if (riscv_isa_extension_available(isainfo->isa, ZBB))
> > > -                       pair->value |= RISCV_HWPROBE_EXT_ZBB;
> > > -               else
> > > -                       missing |= RISCV_HWPROBE_EXT_ZBB;
> > > -
> > > -               if (riscv_isa_extension_available(isainfo->isa, ZBS))
> > > -                       pair->value |= RISCV_HWPROBE_EXT_ZBS;
> > > -               else
> > > -                       missing |= RISCV_HWPROBE_EXT_ZBS;
> > > +#define CHECK_ISA_EXT(__ext)                                                   \
> > > +               do {                                                            \
> > > +                       if (riscv_isa_extension_available(isainfo->isa, __ext)) \
> > > +                               pair->value |= RISCV_HWPROBE_EXT_##__ext;       \
> > > +                       else                                                    \
> > > +                               missing |= RISCV_HWPROBE_EXT_##__ext;           \
> > > +               } while (false)
> > > +
> > > +               /*
> > > +                * Only use CHECK_ISA_EXT() for extensions which can be exposed
> > > +                * to userspace, regardless of the kernel's configuration, as no
> > > +                * other checks, besides presence in the hart_isa bitmap, are
> > > +                * made.
> > 
> > This comment alludes to a dangerous trap, but I'm having trouble
> > understanding what it is.
> 
> 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.

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



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