On 12/21/2015 08:29 AM, Ziviani . wrote:
Hello list!

I'm new here and interested in hot-plug multi-function PCI devices. Basically I'd like to know why Libvirt does not support it. I've been through the archives and basically found this thread:

https://www.redhat.com/archives/libvir-list/2011-May/msg00457.html

But Qemu seems to handle it accordingly:
virsh qemu-monitor-command --hmp fedora-23 'device_add vfio-pci,host=00:16.0,addr=08.0'
virsh qemu-monitor-command --hmp fedora-23 'device_add vfio-pci,host=00:16.3,addr=08.3'

GUEST:
# lspci
(snip)
00:08.0 Communication controller: Intel Corporation 8 Series HECI #0 (rev 04)
00:08.3 Serial controller: Intel Corporation 8 Series HECI KT (rev 04)

However, using Libvirt:

% virsh attach-device fedora-23 pci_0000_00_16_0.xml --live
Device attached successfully

% virsh attach-device fedora-23 pci_0000_00_16_3.xml --live
error: Failed to attach device from pci_0000_00_16_3.xml
error: internal error: Only PCI device addresses with function=0 are supported

I made some changes on domain_addr.c[1] for testing and it worked.


% virsh attach-device fedora-23 pci_0000_00_16_3.xml --live
Device attached successfully

GUEST:
# lspci
(snip)
00:08.0 Communication controller: Intel Corporation 8 Series HECI #0 (rev 04)
00:08.3 Serial controller: Intel Corporation 8 Series HECI KT (rev 04)

So there is more to it that I'm not aware?

You're relying on behavior in the guest OS for which there is no standard (and which, by definition, doesn't work on real hardware, so no guest OS will be expecting it; a friend more familiar with this has told me that probably qemu is sending an (acpi?) "device check" to the guest for each function that is added, and in your case it's apparently "doing the right thing" in response to that). But just because it is successful in this one case doesn't mean that it will be successful in all situations; likely it won't be. So while the qemu monitor takes the laissez-faire approach of allowing you to try it and letting you pick up the pieces when it fails, libvirt prevents it because it is bound to fail, and thus not supportable.

There has recently been some work in qemu to "save up" any requests to attach devices with function > 0, then present them all to the guest at once when function 0 is attached. This is the only standard way to handle hotplug of multiple functions in a slot. Hot unplug can only happen for all functions in the slot at once. I'm not sure of the current status of that work, but once it is in and stable, libvirt will support it.

Thank you!


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