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

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Date:   Wed, 20 Feb 2019 15:44:19 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCHv6 07/10] acpi/hmat: Register processor domain to its
 memory

On Wed, Feb 20, 2019 at 11:21:45PM +0100, Rafael J. Wysocki wrote:
> On Wed, Feb 20, 2019 at 11:11 PM Dave Hansen <dave.hansen@...el.com> wrote:
> > On 2/20/19 2:02 PM, Rafael J. Wysocki wrote:
> > >> diff --git a/drivers/acpi/hmat/Kconfig b/drivers/acpi/hmat/Kconfig
> > >> index c9637e2e7514..08e972ead159 100644
> > >> --- a/drivers/acpi/hmat/Kconfig
> > >> +++ b/drivers/acpi/hmat/Kconfig
> > >> @@ -2,6 +2,7 @@
> > >>  config ACPI_HMAT
> > >>         bool "ACPI Heterogeneous Memory Attribute Table Support"
> > >>         depends on ACPI_NUMA
> > >> +       select HMEM_REPORTING
> > > If you want to do this here, I'm not sure that defining HMEM_REPORTING
> > > as a user-selectable option is a good idea.  In particular, I don't
> > > really think that setting ACPI_HMAT without it makes a lot of sense.
> > > Apart from this, the patch looks reasonable to me.
> >
> > I guess the question is whether we would want to allow folks to consume
> > the HMAT inside the kernel while not reporting it out via
> > HMEM_REPORTING.  We have some in-kernel users of the HMAT lined up like
> > mitigations for memory-side caches.
> >
> > It's certainly possible that folks would want to consume those
> > mitigations without anything in sysfs.  They might not even want or need
> > NUMA support itself, for instance.
> >
> > So, what should we do?
> >
> > config HMEM_REPORTING
> >         bool # no user-visible prompt
> >         default y if ACPI_HMAT
> >
> > So folks can override in their .config, but they don't see a prompt?
> 
> Maybe it would be better to make HMEM_REPORTING do "select ACPI_HMAT if ACPI".
> 
> The mitigations could then do that too if they depend on HMAT and
> ACPI_HMAT need not be user-visible at all.

That sounds okay, though it would create unreachable code if !ACPI since
that's the only user for the new reporting interfaces.