Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP

David Hildenbrand
2020-05-01T09:34:22Z

On 01.05.20 00:24, Andrew Morton wrote:
> On Thu, 30 Apr 2020 20:43:39 +0200 David Hildenbrand <david@redhat.com> wrote:
>
>>>
>>> Why does the firmware map support hotplug entries?
>>
>> I assume:
>>
>> The firmware memmap was added primarily for x86-64 kexec (and still, is
>> mostly used on x86-64 only IIRC). There, we had ACPI hotplug. When DIMMs
>> get hotplugged on real HW, they get added to e820. Same applies to
>> memory added via HyperV balloon (unless memory is unplugged via
>> ballooning and you reboot ... the the e820 is changed as well). I assume
>> we wanted to be able to reflect that, to make kexec look like a real reboot.
>>
>> This worked for a while. Then came dax/kmem. Now comes virtio-mem.
>>
>>
>> But I assume only Andrew can enlighten us.
>>
>> @Andrew, any guidance here? Should we really add all memory to the
>> firmware memmap, even if this contradicts with the existing
>> documentation? (especially, if the actual firmware memmap will *not*
>> contain that memory after a reboot)
>
> For some reason that patch is misattributed - it was authored by
> Shaohui Zheng <shaohui.zheng@intel.com>, who hasn't been heard from in
> a decade. I looked through the email discussion from that time and I'm
> not seeing anything useful. But I wasn't able to locate Dave Hansen's
> review comments.

Okay, thanks for checking. I think the documentation from 2008 is pretty
clear what has to be done here. I will add some of these details to the
patch description.

Also, now that I know that esp. kexec-tools already don't consider
dax/kmem memory properly (memory will not get dumped via kdump) and
won't really suffer from a name change in /proc/iomem, I will go back to
the MHP_DRIVER_MANAGED approach and
1. Don't create firmware memmap entries
2. Name the resource "System RAM (driver managed)"
3. Flag the resource via something like IORESOURCE_MEM_DRIVER_MANAGED.

This way, kernel users and user space can figure out that this memory
has different semantics and handle it accordingly - I think that was
what Eric was asking for.

Of course, open for suggestions.

--
Thanks,

David / dhildenb