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Date:   Mon, 25 Feb 2019 23:30:21 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Keith Busch <keith.busch@...el.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCHv6 07/10] acpi/hmat: Register processor domain to its memory

On Mon, Feb 25, 2019 at 5:51 PM Keith Busch <keith.busch@...el.com> wrote:
>
> On Sun, Feb 24, 2019 at 08:59:45PM +0100, Rafael J. Wysocki wrote:
> > On Fri, Feb 22, 2019 at 7:48 PM Keith Busch <keith.busch@...el.com> wrote:
> > > If I do it the other way around, that's going to make HMEM_REPORTING
> > > complicated if a non-ACPI implementation wants to report HMEM
> > > properties.
> >
> > But the mitigations that Dave was talking about get in the way, don't they?
> >
> > Say there is another Kconfig option,CACHE_MITIGATIONS, to enable them.
> > Then you want ACPI_HMAT to be set when that it set and you also want
> > ACPI_HMAT to be set when HMEM_REPORTING and ACPI_NUMA are both set.
> >
> > OTOH, you may not want HMEM_REPORTING to be set when CACHE_MITIGATIONS
> > is set, but that causes ACPI_HMAT to be set and which means that
> > ACPI_HMAT alone will not be sufficient to determine the
> > HMEM_REPORTING value.
>
> I can't think of when we'd want to suppress reporting these attributes
> to user space, but I can split HMAT enabling so it doesn't depend on
> HMEM_REPORTING just in case there really is an in-kernel user that
> definitely does not want the same attributes exported.

I'd rather simplify HMAT enabling than make it more complicated, so
splitting it would be worse than what you have already IMO.

> > Now, if you prompt for HMEM_REPORTING and make it depend on ACPI_NUMA,
> > then ACPI_HMAT can be selected by that (regardless of the
> > CACHE_MITIGATIONS value).
> >
> > And if someone wants to use HMEM_REPORTING without ACPI_NUMA, it can
> > be made depend on whatever new option is there for that non-ACPI
> > mechanism.
> >
> > There might be a problem if someone wanted to enable the alternative
> > way of HMEM_REPORTING if ACPI_NUMA was set (in which case HMAT would
> > have to be ignored even if it was present), but in that case there
> > would need to be an explicit way to choose between HMAT and non-HMAT
> > anyway.
> >
> > In any case, I prefer providers to be selected by consumers and not
> > the other way around, in case there are multiple consumers for one
> > provider.
>
> Well, the HMEM_REPORTING fundamentally has no dependency on any of these
> things and I've put some effort into making this part provider agnostic.

Which I agree with.

> I will change it if this concern is gating acceptance, but I don't
> think it's as intuitive for generic interfaces to be the selector for
> implementation specific providers.

That is sort of a chicken-and-egg issue about what is more fundamental
that could be discussed forever. :-)

My original point was that if you regard ACPI_HMAT as the more
fundamental thing, then you should prompt for it and select
HMEM_REPORTING automatically from there.  Or the other way around if
you regard HMEM_REPORTING as more fundamental.  Prompting for both of
them may lead to issues.

As long as that is taken into account, I'm basically fine with any of
the two choices.

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