On 18/10/2023 19:45, Evan Green wrote:
> On Wed, Oct 18, 2023 at 10:37 AM Conor Dooley <conor@xxxxxxxxxx> 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>>>>> ---
>>>>> 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
