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From: Dan Williams <dan.j.williams@intel.com>
To: David Hildenbrand <david@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	virtio-dev@lists.oasis-open.org,
	virtualization@lists.linux-foundation.org,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Linux ACPI <linux-acpi@vger.kernel.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	linux-hyperv@vger.kernel.org,
	linux-s390 <linux-s390@vger.kernel.org>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Michal Hocko <mhocko@kernel.org>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	Michal Hocko <mhocko@suse.com>,
	Pankaj Gupta <pankaj.gupta.linux@gmail.com>,
	Wei Yang <richard.weiyang@gmail.com>, Baoquan He <bhe@redhat.com>
Subject: Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP
Date: Fri, 1 May 2020 09:56:56 -0700
Message-ID: <CAPcyv4j=YKnr1HW4OhAmpzbuKjtfP7FdAn4-V7uA=b-Tcpfu+A@mail.gmail.com> (raw)
In-Reply-To: <5c908ec3-9495-531e-9291-cbab24f292d6@redhat.com>

On Fri, May 1, 2020 at 2:34 AM David Hildenbrand <david@redhat.com> wrote:
>
> 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.

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?

  reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-30 10:29 [PATCH v2 0/3] mm/memory_hotplug: Allow to not create firmware memmap entries David Hildenbrand
2020-04-30 10:29 ` [PATCH v2 1/3] mm/memory_hotplug: Prepare passing flags to add_memory() and friends David Hildenbrand
2020-04-30 10:29 ` [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP David Hildenbrand
2020-04-30 15:38   ` Eric W. Biederman
2020-04-30 15:52     ` David Hildenbrand
2020-04-30 16:04       ` Dave Hansen
2020-04-30 16:33       ` Eric W. Biederman
2020-04-30 16:49         ` David Hildenbrand
2020-04-30 18:06           ` Eric W. Biederman
2020-04-30 18:43             ` David Hildenbrand
2020-04-30 18:58               ` Dan Williams
2020-04-30 22:24               ` Andrew Morton
2020-05-01  9:34                 ` David Hildenbrand
2020-05-01 16:56                   ` Dan Williams [this message]
2020-05-01 17:21                     ` David Hildenbrand
2020-05-01 17:39                       ` Dan Williams
2020-05-01 17:45                         ` David Hildenbrand
2020-05-01 17:51                           ` David Hildenbrand
2020-05-01 18:03                             ` Dan Williams
2020-05-01 18:14                               ` David Hildenbrand
2020-05-01 18:43                                 ` Dan Williams
2020-05-01 19:17                                   ` David Hildenbrand
2020-05-01 20:12                                     ` Dan Williams
2020-05-01 21:10                                       ` David Hildenbrand
2020-05-01 21:52                                         ` Dan Williams
2020-05-02  9:26                                           ` David Hildenbrand
2020-05-02 18:03                                             ` Dan Williams
2020-04-30 10:29 ` [PATCH v2 3/3] device-dax: Add system ram (add_memory()) with MHP_NO_FIRMWARE_MEMMAP David Hildenbrand
2020-04-30 11:23   ` Dave Hansen
2020-04-30 15:28     ` David Hildenbrand

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