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

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Message-ID: <20231019-flatten-showbiz-127b2e917a7a@spud>
Date:   Thu, 19 Oct 2023 11:22:26 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Clément Léger <cleger@...osinc.com>
Cc:     Evan Green <evan@...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 Thu, Oct 19, 2023 at 09:26:31AM +0200, Clément Léger wrote:
> 
> 
> On 18/10/2023 19:36, Conor Dooley wrote:
> > 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.

> Yes, the KConfig checks are already done by the headers, adding #ifdef
> would be redundant even if more coherent with the comment

I don't really understand what the first part of this means, or why using
avoidable ifdeffery here would be desirable.

> BTW, wouldn't
> it make more sense to get rid out of the unsupported extensions directly
> at ISA string parsing ? ie, if kernel is compiled without V support,
> then do not set the bits corresponding to these in the riscv_isa_ext[]
> array ? But the initial intent was probably to be able to report the
> full string through cpuinfo.

Yeah, hysterical raisins I guess, it's always been that way. I don't
think anyone originally thought about such configurations and that is
how the cpuinfo stuff behaves. I strongly dislike the
riscv_isa_extension_available() interface, but one of Drew's patches
does at least improve things a bit. Kinda waiting for some of the
patches in flight to settle down before deciding if I want to refactor
stuff to be less of a potential for shooting oneself in the foot.

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