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

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Message-ID: <20231018-scrap-bankable-a0f321d97a46@spud>
Date:   Wed, 18 Oct 2023 18:33:34 +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 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.

> 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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