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

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Message-ID: <3dfde5e3-e1e2-2097-afa1-303042de5e07@redhat.com>
Date:   Fri, 21 Aug 2020 20:30:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.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>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
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        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 00/23] device-dax: Support sub-dividing soft-reserved
 ranges

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? (AFAIK,
"soft-reserved" will be visible in /proc/iomem, or am I confusing
stuff?) IOW, it "performance differentiated" is not universally
applicable, maybe  "specific purpose memory" is ?

-- 
Thanks,

David / dhildenb