On 8/25/20 8:56 AM, Michal Privoznik wrote:
On 8/25/20 1:40 PM, Henry lol wrote:
Hi guys,
I'm wondering whether libvirt supports live migration for the VM with
PCI passthrough devices.
or it must be assumed before live migration that all passthrough
devices be unplugged?
Unfortunately, this is still not supported. The problem is that PCI
devices themselves are not capable of dumping their internal state and
restoring from it on destination.
There is a long thread started last month that discuss what the
interface should look like, but at this point I guess we are still far
away from it:
https://www.redhat.com/archives/libvir-list/2020-July/msg00675.html
If so, all unplugged devices should be manually hot-plugged to the VM
after migration??
This is the usual mode of operation, yes.
There is one exception to this - if the device to be migrated is the VF
of an SRIOV-capable network card, then there is machinery in qemu to
automatically unplug the device before migration starts, and then
automatically plug in a new similar device on the other end after the
guest has started there.
This all works in concert with a virtio-net emulated network device that
is bonded together in the guest (linux only) to provide a single network
device in the guest - when the VF is visible and working, it will be
preferred for all traffic, but if the VF is missing then the virtio-net
emulated device will be used.
The result is that under normal operation the vfio-assigned VF is used
for all network traffic, but during migration when the VF has been
unplugged, the virtio-net device is used, so there is no disruption in
network traffic, just a bit of degraded performance during migration.
Here is a description of how this is configured in a libvirt domain
definition:
https://www.libvirt.org/formatdomain.html#teaming-a-virtio-hostdev-nic-pair
This is all fairly new, and unfortunately doesn't work properly for all
SRIOV cards.
