linux-kernel - Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE

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Date:   Thu, 30 Apr 2020 15:24:03 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtio-dev@...ts.oasis-open.org,
        virtualization@...ts.linux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, linux-acpi@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-hyperv@...r.kernel.org,
        linux-s390@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Michal Hocko <mhocko@...nel.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Baoquan He <bhe@...hat.com>
Subject: Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce
 MHP_NO_FIRMWARE_MEMMAP

On Thu, 30 Apr 2020 20:43:39 +0200 David Hildenbrand <david@...hat.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@...el.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.