[libvirt] QEMU -M nvdimm=on and hotplug

[Stefan Hajnoczi]

Hi Michal,
You asked how NVDIMM hotplug should work if the domain was launched
without -M nvdimm=on.  I'm not very familiar with ACPI so this may be
incorrect.

Hotplug relies on the presence of ACPI tables (SSDT Device=NVDR
_HID=ACPI0012) added by -M nvdimm=on.  The ACPI tables contain the
interface needed to handle hotplug events.

The device_add command fails with the following message if -M nvdimm=on
is missing:

  nvdimm is not enabled: missing 'nvdimm' in '-M'

I'm not sure if ACPI allows SSDT tables to be dynamically added but in
any case the current QEMU code doesn't support that.

Options for libvirt:

1. Rely on QEMU's device_add error.
   Pro: If QEMU behavior ever changes then libvirt will also start
        working without code changes.
   Cons: Does libvirt interpret QMP error strings?

2. Only allow NVDIMM hotplug if the domain was started with -M
   nvdimm=on.

I think QEMU will not add -M nvdimm=on to the "pc" machine type by
default since it adds the NVDIMM DSM hardware interface that increases
the security attack surface.

Stefan