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Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP
- To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
- From: David Hildenbrand <david@xxxxxxxxxx>
- Date: Fri, 1 May 2020 11:34:22 +0200
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- Cc: virtio-dev@xxxxxxxxxxxxxxxxxxxx, linux-hyperv@xxxxxxxxxxxxxxx, Michal Hocko <mhocko@xxxxxxxx>, Baoquan He <bhe@xxxxxxxxxx>, linux-acpi@xxxxxxxxxxxxxxx, Wei Yang <richard.weiyang@xxxxxxxxx>, linux-s390@xxxxxxxxxxxxxxx, linux-nvdimm@xxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, "Michael S . Tsirkin" <mst@xxxxxxxxxx>, "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>, Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Michal Hocko <mhocko@xxxxxxxxxx>, linuxppc-dev@xxxxxxxxxxxxxxxx
- Delivery-date: Fri, 01 May 2020 09:35:00 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
--
Thanks,
David / dhildenb
|
