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

[Dr. David Alan Gilbert]

* David Hildenbrand (address@hidden) wrote:
> On 21/09/2018 18:55, Dr. David Alan Gilbert wrote:
> > * 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?
> 
> My opinion is: easier.
> 
> Being an acpi device, we would have devices that are both, ACPI and
> virtio. Or you have to create separate device and somehow link them
> together (both in QEMU and in Linux)
> 
> I don't like gluing virtio devices to different, unrelated technologies.
> Especially, you could never support architectures that don't support
> ACPI. (with virtio-pmem, we could eventually even support architectures
> that don't support something like nvdimm or acpi).
> 
> > 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?
> 
> As far as I remember, booting from NVDIMM is not implemented anywhere. I
> guess if it would be, we could also realize booting from virtio-pmem
> (e.g. via grub extension).

OK, that's fair enough; I was just curious about why there was the
difference.

Dave

> > 
> > Dave
> 
> 
> -- 
> 
> Thanks,
> 
> David / dhildenb
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK