linux-kernel - Re: [PATCHv6 07/10] acpi/hmat: Register processor domain to its memory

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Date:   Mon, 25 Feb 2019 09:51:18 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     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 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.

> 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.
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.