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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 23:43:50 +0200
Message-Id: <D0A25288-C72D-47C1-BD8F-C1C1A6BFA0BF@redhat.com>
References: <646DDE9B-90C2-493A-958C-90EFA1CCA475@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Ira Weiny <ira.weiny@intel.com>,
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In-Reply-To: <646DDE9B-90C2-493A-958C-90EFA1CCA475@redhat.com>
To:     Dan Williams <dan.j.williams@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> Am 21.08.2020 um 23:34 schrieb David Hildenbrand <david@redhat.com>:
>=20
> =EF=BB=BF
>=20
>>> 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@redha=
t.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> wr=
ote:
>>>>>=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 sinc=
e v5.5:
>>=20
>>  262b45ae3ab4 x86/efi: EFI soft reservation to E820 enumeration
>>=20
>=20
> I was asking because I was struggling to even understand what =E2=80=9Esof=
t-reserved=E2=80=9C is and I could bet most people have no clue what that is=
 supposed to be.
>=20
> In contrast =E2=80=9Epersistent memory=E2=80=9C or =E2=80=9Especial purpos=
e memory=E2=80=9C in /proc/iomem is something normal (Linux using) human bei=
ngs can understand.

Obviously s/normal/most/

Cheers!=