Date: Tue, 14 May 2024 13:43:54 +0100
From: Conor Dooley <conor@...nel.org>
To: Clément Léger <cleger@...osinc.com>
Cc: Conor Dooley <conor.dooley@...rochip.com>,
	Jonathan Corbet <corbet@....net>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Anup Patel <anup@...infault.org>, Shuah Khan <shuah@...nel.org>,
	Atish Patra <atishp@...shpatra.org>, linux-doc@...r.kernel.org,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, kvm@...r.kernel.org,
	kvm-riscv@...ts.infradead.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 02/11] riscv: add ISA extensions validation

On Tue, May 14, 2024 at 09:53:08AM +0200, Clément Léger wrote:
> 
> 
> 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 ?

I realised last night that I'd not got back to this thread and meant to
do that today (I had accidentally deleted it from my mailbox), but I had
a migraine this morning and so didn't.
I did bring it up and IIRC Palmer was of the opinion that we should try
our best to infer extensions.

> > 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 ?

No, with the above excuse. I'll try to get to it today or tomorrow...

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