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From:   David Hildenbrand <david@redhat.com>
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Subject: Re: [PATCH v4 00/23] device-dax: Support sub-dividing soft-reserved ranges
Date:   Fri, 21 Aug 2020 23:33:57 +0200
Message-Id: <646DDE9B-90C2-493A-958C-90EFA1CCA475@redhat.com>
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Cc:     David Hildenbrand <david@redhat.com>,
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In-Reply-To: <CAPcyv4gTJgZ0jM3oRM8Exs7MKwyNHF5yWNceAFrX7k8KfFcBig@mail.gmail.com>
To:     Dan Williams <dan.j.williams@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> Am 21.08.2020 um 23:17 schrieb Dan Williams <dan.j.williams@intel.com>:
>=20
> =EF=BB=BFOn Fri, Aug 21, 2020 at 11:30 AM David Hildenbrand <david@redhat.=
com> wrote:
>>=20
>>> On 21.08.20 20:27, Dan Williams wrote:
>>> On Fri, Aug 21, 2020 at 3:15 AM David Hildenbrand <david@redhat.com> wro=
te:
>>>>=20
>>>>>>=20
>>>>>> 1. On x86-64, e820 indicates "soft-reserved" memory. This memory is n=
ot
>>>>>> 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?
>>>>>=20
>>>>> 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.
>>>>=20
>>>> 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, n=
o?
>>>=20
>>> 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.
>>=20
>> Okay, still "soft-reserved" is x86-64 specific, no?
>=20
> There's nothing preventing other EFI archs, or a similar designation
> in another firmware spec, picking up this policy.
>=20
>>  (AFAIK,
>> "soft-reserved" will be visible in /proc/iomem, or am I confusing
>> stuff?)
>=20
> No, you're correct.
>=20
>> IOW, it "performance differentiated" is not universally
>> applicable, maybe  "specific purpose memory" is ?
>=20
> Those bikeshed colors don't seem an improvement to me.
>=20
> "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.
>=20
> Also note that the current color of the bikeshed has already shipped since=
 v5.5:
>=20
>   262b45ae3ab4 x86/efi: EFI soft reservation to E820 enumeration
>=20

I was asking because I was struggling to even understand what =E2=80=9Esoft-=
reserved=E2=80=9C is and I could bet most people have no clue what that is s=
upposed to be.

In contrast =E2=80=9Epersistent memory=E2=80=9C or =E2=80=9Especial purpose m=
emory=E2=80=9C in /proc/iomem is something normal (Linux using) human beings=
 can understand.

But anyhow, just details, and you=E2=80=98re telling me that that ship alrea=
dy sailed. So no further comments from my side.

Thanks for all the info!=