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

[David Hildenbrand <david@redhat.com>]

> Am 21.08.2020 um 23:34 schrieb David Hildenbrand <david@redhat.com>:
> 
> 
> 
>>> Am 21.08.2020 um 23:17 schrieb Dan Williams <dan.j.williams@intel.com>:
>>> 
>>> On Fri, Aug 21, 2020 at 11:30 AM David Hildenbrand <david@redhat.com> wrote:
>>> 
>>>> On 21.08.20 20:27, Dan Williams wrote:
>>>> On Fri, Aug 21, 2020 at 3:15 AM David Hildenbrand <david@redhat.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
>> 
> 
> I was asking because I was struggling to even understand what „soft-reserved“ is and I could bet most people have no clue what that is supposed to be.
> 
> In contrast „persistent memory“ or „special purpose memory“ in /proc/iomem is something normal (Linux using) human beings can understand.

s/normal/most/ of course :)
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