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

Andrew Morton
2020-04-30T22:24:04Z

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.