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

Conor Dooley
2024-04-30T11:45:23Z

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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.

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.

Cheers,
Conor.

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