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

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Date:   Thu, 19 Oct 2023 11:46:51 +0200
From:   Clément Léger <cleger@...osinc.com>
To:     Evan Green <evan@...osinc.com>, Conor Dooley <conor@...nel.org>
Cc:     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 18/10/2023 19:45, Evan Green wrote:
> On Wed, Oct 18, 2023 at 10:37 AM Conor Dooley <conor@...nel.org> 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.
> 
> 
> Ohh, got it. The word "can" is doing a lot of heavy lifting in that
> comment. So maybe something like: "This macro performs little in the
> way of extension-specific kernel readiness checks. It's assumed other
> gating factors like required Kconfig settings have already been
> confirmed to support exposing the given extension to usermode". ...
> But, you know, make it sparkle.

Hi Even,

Indeed the comment was a bit misleading, is this more clear ?

/*
 * Only use CHECK_ISA_EXT() for extensions which are usable by
 * userspace with respect to the kernel current configuration.
 * For instance, ISA extensions that uses float operations
 * should not be exposed when CONFIG_FPU is not set.
 */

Clément

> 
> -Evan