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

["Clément Léger" <cleger@rivosinc.com>]

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@rivosinc.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@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_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. 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.

Clément

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