[libvirt] Regarding PCI/PCIe Multifunction hotplug support in libvirt

[Shivaprasad bhat]

Hi All,

I am exploring how to go about supporting multi-function hot-plug/unplug
support from libvirt now that Qemu has enabled it.

The Qemu semantics is that, qemu queues the notification to guest until
function 0 is hot-
plugged and on function 0 hot-plug, all the previously added devices in the
slot are
notified to the guest.
For hot-unplug - On x86, unplug of any device in the slot would unplug all
the functions of
the slot. On PPC, unplug all non-zero functions first, then unplug the
function zero which
triggers the unplug of all the functions in the slot.

On Libvirt, I had a brief chat with Laine Stump on IRC and we "think" the
below semantics
would be appropriate.
1) Change the virDomainAttachDeviceFlags() to recognize multiple devices in
the XML
and the application should make one call to attach all the functions for
the slot at one
time.
2) The libvirt qemu driver to translate this into multiple attach device
commands to qemu
with the final operation being for function 0.
3) The XML now needs to accept multiple devices and there is a need for
single parent
element. I am thinking if we should have all devices to be enclosed in
<device></device>
parent element. Note that this is only when all the devices should go to
the same slot. We
probably should disallow user attempting to hot-plug to different slots
with this.
4) For the hot-unplug, the application to send all the devices the same way
enclosed in
<device></devices> and libvirt to go ahead with unplug only if all the
devices are
specified in the XML for the slot.

Want to know if you foresee any problem with using <device></device>
semantics Or you
have any different approach/suggestions. Any comments greatly appreciated.

Thanks and Regards,
Shivaprasad
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