Re: [libvirt] Hot plug multi function PCI devices
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!
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