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

[Dan Williams]

On Fri, May 1, 2020 at 2:34 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 01.05.20 00:24, Andrew Morton wrote:
> > On Thu, 30 Apr 2020 20:43:39 +0200 David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxx>, 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.

I'm still more of a fan of this being communicated by "System RAM"
being parented especially because that tells you something about how
the memory is driver-managed and which mechanism might be in play.
What about adding an optional /sys/firmware/memmap/X/parent attribute.
This lets tooling check if it cares via that interface and lets it
lookup the related infrastructure to interact with if it would do
something different for virtio-mem vs dax/kmem?