Re: [PATCH v4 02/11] riscv: add ISA extensions validation

Clément Léger
2024-05-14T07:53:11Z

On 30/04/2024 13:44, Conor Dooley wrote:
> On Tue, Apr 30, 2024 at 09:18:47AM +0200, Clément Léger wrote:
>>
>>
>> On 30/04/2024 00:15, Conor Dooley wrote:
>>> On Mon, Apr 29, 2024 at 05:04:55PM +0200, Clément Léger wrote:
>>>> Since a few extensions (Zicbom/Zicboz) already needs validation and
>>>> future ones will need it as well (Zc*) add a validate() callback to
>>>> struct riscv_isa_ext_data. This require to rework the way extensions are
>>>> parsed and split it in two phases. First phase is isa string or isa
>>>> extension list parsing and consists in enabling all the extensions in a
>>>> temporary bitmask without any validation. The second step "resolves" the
>>>> final isa bitmap, handling potential missing dependencies. The mechanism
>>>> is quite simple and simply validate each extension described in the
>>>> temporary bitmap before enabling it in the final isa bitmap. validate()
>>>> callbacks can return either 0 for success, -EPROBEDEFER if extension
>>>> needs to be validated again at next loop. A previous ISA bitmap is kept
>>>> to avoid looping mutliple times if an extension dependencies are never
>>>> satisfied until we reach a stable state. In order to avoid any potential
>>>> infinite looping, allow looping a maximum of the number of extension we
>>>> handle. Zicboz and Zicbom extensions are modified to use this validation
>>>> mechanism.
>>>
>>> Your reply to my last review only talked about part of my comments,
>>> which is usually what you do when you're gonna implement the rest, but
>>> you haven't.
>>> I like the change you've made to shorten looping, but I'd at least like
>>> a response to why a split is not worth doing :)
>>
>> Hi Conor,
>>
>> Missed that point since I was feeling that my solution actually
>> addresses your concerns. Your argument was that there is no reason to
>> loop for Zicbom/Zicboz but that would also apply to Zcf in case we are
>> on RV64 as well (since zcf is not supported on RV64). So for Zcf, that
>> would lead to using both mecanism or additional ifdefery with little to
>> no added value since the current solution actually solves both cases:
>>
>> - We don't have any extra looping if all validation callback returns 0
>> (except the initial one on riscv_isa_ext, which is kind of unavoidable).
>> - Zicbom, Zicboz callbacks will be called only once (which was one of
>> your concern).
>>
>> Adding a second kind of callback for after loop validation would only
>> lead to a bunch of additional macros/ifdefery for extensions with
>> validate() callback, with validate_end() or with both (ie Zcf)). For
>> these reasons, I do not think there is a need for a separate mechanism
>> nor additional callback for such extensions except adding extra code
>> with no real added functionality.
>>
>> AFAIK, the platform driver probing mechanism works the same, the probe()
>> callback is actually called even if for some reason properties are
>> missing from nodes for platform devices and thus the probe() returns
>> -EINVAL or whatever.
>>
>> Hope this answers your question,
>
> Yeah, pretty much I am happy with just an "it's not worth doing it"
> response. Given it wasn't your first choice, I doubt you're overly happy
> with it either, but I really would like to avoid looping to closure to
> sort out dependencies - particularly on the boot CPU before we bring
> anyone else up, but if the code is now more proactive about breaking
> out, I suppose that'll have to do :)
> I kinda wish we didn't do this at all, but I think we've brought this
> upon ourselves via hwprobe. I'm still on the fence as to whether things
> that are implied need to be handled in this way. I think I'll bring this
> up tomorrow at the weekly call, because so far it's only been you and I
> discussing this really and it's a policy decision that hwprobe-ists
> should be involved in I think.

Hi Conor,

Were you able to discuss that topic ?

>
> Implied extensions aside, I think we will eventually need this stuff
> anyway, for extensions that make no sense to consider if a config option
> for a dependency is disabled.
> From talking to Eric Biggers the other week about
> riscv_isa_extension_available() I'm of the opinion that we need to do
> better with that interface w.r.t. extension and config dependencies,
> and what seems like a good idea to me at the moment is putting tests for
> IS_ENABLED(RISCV_ISA_FOO) into these validate hooks.
>
> I'll try to look at the actual implementation here tomorrow.

Did you found time to look at the implementation ?

Thanks,

Clément

>
> Cheers,
> Conor.