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Subject: Re: [PATCH 0/7] ACPI HMAT memory sysfs representation
To:     Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>
Cc:     Keith Busch <keith.busch@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>
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From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <325d0e69-053a-ae9c-eede-7cdf28b1dbd6@arm.com>
Date:   Tue, 27 Nov 2018 15:45:40 +0530
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On 11/26/2018 11:38 PM, Dan Williams wrote:
> On Mon, Nov 26, 2018 at 8:42 AM Dave Hansen <dave.hansen@intel.com> wrote:
>>
>> On 11/23/18 1:13 PM, Dan Williams wrote:
>>>> A new system call makes total sense to me.  I have the same concern
>>>> about the completeness of what's exposed in sysfs, I just don't see a
>>>> _route_ to completeness with sysfs itself.  Thus, the minimalist
>>>> approach as a first step.
>>> Outside of platform-firmware-id to Linux-numa-node-id what other
>>> userspace API infrastructure does the kernel need to provide? It seems
>>> userspace enumeration of memory attributes is fully enabled once the
>>> firmware-to-Linux identification is established.
>>
>> It would be nice not to have each app need to know about each specific
>> platform's firmware.
> 
> The app wouldn't need to know if it uses a common library. Whether the
> library calls into the kernel or not is an implementation detail. If
> it is information that only the app cares about and the kernel does
> not consume, why have a syscall?

If we just care about platform-firmware-id <--> Linux-numa-node-id mapping
and fetching memory attribute from the platform (and hiding implementation
details in a library) then the following interface should be sufficient.

/sys/devices/system/node/nodeX/platform_id

But as the series proposes (and rightly so) kernel needs to start providing
ABI interfaces for memory attributes instead of hiding them in libraries.