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Re: [Qemu-devel] [PATCH v3 22/22] pc: support for virtio-pmem

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH v3 22/22] pc: support for virtio-pmem
Date: Fri, 21 Sep 2018 17:55:16 +0100
User-agent: Mutt/1.10.1 (2018-07-13)

* David Hildenbrand (address@hidden) wrote:
> On 21/09/2018 18:00, Dr. David Alan Gilbert wrote:
> > * David Hildenbrand (address@hidden) wrote:
> >> Compile virtio-pmem for x86 and properly hotplug virtio-pmem devices
> >> (memory-device part) from our pc machine hotplug handler.
> >>
> >> Signed-off-by: David Hildenbrand <address@hidden>
> > 
> > I see a few other places in the pc subdir that have checks for
> > TYPE_NVDIMM; can you just explain why they are untouched:
> >   i386/pc.c: pc_memory_unplug_request
> I think I had an answer why that isn't needed, but for unplug we might
> still be missing something.
> In general: unplug requests are always directed at the proxy device (via
> qdev_unplug()) (e.g. virtio-pmem-pci, see below). virtio-pmem is a child
> device of virtio-pmem-pci. Once virtio-pmem-pci is finally unrealized,
> the virtio-mem device will be unrealized.
> Now, there are no hotplug handlers getting called as far as I can see
> for that child device. there would have to be an proper call to a
> hotplug handler somewhere when setting the device to unrealized. (and
> that's the place where a unplug requests does not make really sense, but
> only a straight unplug - because we are half way through removing the
> hierarchy of devices - one of the reason why also failures of unrealize
> are usually ignored).
> I'll look into the details. Thanks for pointing that out, this stuff
> always messes with my head.
> >   acpi/ich9.c: ich9_pm_device_plug_cb
> >   acpi/piix4.c:piix4_device_plug_cb
> virtio-pmem is not an acpi device, that's why these don't apply.
> E.g. if virtio-pmem-pci is plugged, it will most probably trigger these
> hotplug handlers for the proxy virtio pci device. The proxy device
> creates and realized the virtio-mem (memory-device) - without any
> hotplug handlers involved. That#s the point where we can jump in and
> realize the device via the machine instead.

Does not being an acpi-device make life easier or harder? Again, not
knowing anything about the pmem acpi; if it looked like a normal nvdimm
through ACPI would OSs be able to boot off it?


> > 
> > (Not that I understand the detail of those paths, but I just
> > followed where the existing nvdimm code goes)
> > 
> > Dave
> -- 
> Thanks,
> David / dhildenb
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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