[libvirt] Hot plug multi function PCI devices

[Laine Stump]

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.
>
> [1]https://gist.github.com/jrziviani/1da184c7fd0b413e0426
>
> % 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!
>
>
> --
> libvir-list mailing list
> libvir-list at redhat.com
> https://www.redhat.com/mailman/listinfo/libvir-list

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/libvir-list/attachments/20151221/2743203b/attachment-0001.htm>