linux-kernel - Re: [PATCH v4 00/23] device-dax: Support sub-dividing soft-reserved ranges

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Message-ID: <CAPcyv4gTJgZ0jM3oRM8Exs7MKwyNHF5yWNceAFrX7k8KfFcBig@mail.gmail.com>
Date:   Fri, 21 Aug 2020 14:17:18 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
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        Vishal Verma <vishal.l.verma@...el.com>,
        David Airlie <airlied@...ux.ie>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Joao Martins <joao.m.martins@...cle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Wei Yang <richardw.yang@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
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        <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 00/23] device-dax: Support sub-dividing soft-reserved ranges

On Fri, Aug 21, 2020 at 11:30 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 21.08.20 20:27, Dan Williams wrote:
> > On Fri, Aug 21, 2020 at 3:15 AM David Hildenbrand <david@...hat.com> wrote:
> >>
> >>>>
> >>>> 1. On x86-64, e820 indicates "soft-reserved" memory. This memory is not
> >>>> automatically used in the buddy during boot, but remains untouched
> >>>> (similar to pmem). But as it involves ACPI as well, it could also be
> >>>> used on arm64 (-e820), correct?
> >>>
> >>> Correct, arm64 also gets the EFI support for enumerating memory this
> >>> way. However, I would clarify that whether soft-reserved is given to
> >>> the buddy allocator by default or not is the kernel's policy choice,
> >>> "buddy-by-default" is ok and is what will happen anyways with older
> >>> kernels on platforms that enumerate a memory range this way.
> >>
> >> Is "soft-reserved" then the right terminology for that? It sounds very
> >> x86-64/e820 specific. Maybe a compressed for of "performance
> >> differentiated memory" might be a better fit to expose to user space, no?
> >
> > No. The EFI "Specific Purpose" bit is an attribute independent of
> > e820, it's x86-Linux that entangles those together. There is no
> > requirement for platform firmware to use that designation even for
> > drastic performance differentiation between ranges, and conversely
> > there is no requirement that memory *with* that designation has any
> > performance difference compared to the default memory pool. So it
> > really is a reservation policy about a memory range to keep out of the
> > buddy allocator by default.
>
> Okay, still "soft-reserved" is x86-64 specific, no?

There's nothing preventing other EFI archs, or a similar designation
in another firmware spec, picking up this policy.

>   (AFAIK,
> "soft-reserved" will be visible in /proc/iomem, or am I confusing
> stuff?)

No, you're correct.

> IOW, it "performance differentiated" is not universally
> applicable, maybe  "specific purpose memory" is ?

Those bikeshed colors don't seem an improvement to me.

"Soft-reserved" actually tells you something about the kernel policy
for the memory. The criticism of "specific purpose" that led to
calling it "soft-reserved" in Linux is the fact that "specific" is
undefined as far as the firmware knows, and "specific" may have
different applications based on the platform user. "Soft-reserved"
like "Reserved" tells you that a driver policy might be in play for
that memory.

Also note that the current color of the bikeshed has already shipped since v5.5:

   262b45ae3ab4 x86/efi: EFI soft reservation to E820 enumeration