On 21.08.20 20:27, Dan Williams wrote:
On Fri, Aug 21, 2020 at 3:15 AM David Hildenbrand
>>> 1. On x86-64, e820 indicates "soft-reserved" memory. This memory is
>>> 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 ?
David / dhildenb