This is another attempt at implementing the host side of the net_failover concept (https://www.kernel.org/doc/html/latest/networking/net_failover.html)
The general idea is that we have a pair of devices, a vfio-pci and a emulated device. Before migration the vfio device is unplugged and data flows to the emulated device, on the target side another vfio-pci device is plugged in to take over the data-path. In the guest the net_failover module will pair net devices with the same MAC address. * In the first patch the infrastructure for hiding the device is added for the qbus and qdev APIs. A "hidden" boolean is added to the device state and it is set based on a callback to the standby device which registers itself for handling the assessment: "should the primary device be hidden?" by cross validating the ids of the devices. * In the second patch the virtio-net uses the API to hide the vfio device and unhides it when the feature is acked. Previous discussion: https://patchwork.ozlabs.org/cover/989098/ To summarize concerns/feedback from previous discussion: 1.- guest OS can reject or worse _delay_ unplug by any amount of time. Migration might get stuck for unpredictable time with unclear reason. This approach combines two tricky things, hot/unplug and migration. -> We can surprise-remove the PCI device and in QEMU we can do all necessary rollbacks transparent to management software. Will it be easy, probably not. 2. PCI devices are a precious ressource. The primary device should never be added to QEMU if it won't be used by guest instead of hiding it in QEMU. -> We only hotplug the device when the standby feature bit was negotiated. We save the device cmdline options until we need it for qdev_device_add() Hiding a device can be a useful concept to model. For example a pci device in a powered-off slot could be marked as hidden until the slot is powered on (mst). 3. Management layer software should handle this. Open Stack already has components/code to handle unplug/replug VFIO devices and metadata to provide to the guest for detecting which devices should be paired. -> An approach that includes all software from firmware to higher-level management software wasn't tried in the last years. This is an attempt to keep it simple and contained in QEMU as much as possible. 4. Hotplugging a device and then making it part of a failover setup is not possible -> addressed by extending qdev hotplug functions to check for hidden attribute, so e.g. device_add can be used to plug a device. There are still some open issues: Migration: I'm looking for something like a pre-migration hook that I could use to unplug the vfio-pci device. I tried with a migration notifier but it is called to late, i.e. after migration is aborted due to vfio-pci marked unmigrateable. I worked around this by setting it to migrateable and used a migration notifier on the virtio-net device. Commandline: There is a dependency between vfio-pci and virtio-net devices. One points to the other via new parameters primar=<primary qdev id> and standby='<standby qdev id>'. This means that the primary device needs to be specified after standby device on the qemu command line. Not sure how to solve this. Error handling: Patches don't cover all possible error scenarios yet. I have tested this with a mlx5 NIC and was able to migrate the VM with above mentioned workarounds for open problems. Command line example: qemu-system-x86_64 -enable-kvm -m 3072 -smp 3 \ -machine q35,kernel-irqchip=split -cpu host \ -k fr \ -serial stdio \ -net none \ -qmp unix:/tmp/qmp.socket,server,nowait \ -monitor telnet:127.0.0.1:5555,server,nowait \ -device pcie-root-port,id=root0,multifunction=on,chassis=0,addr=0xa \ -device pcie-root-port,id=root1,bus=pcie.0,chassis=1 \ -device pcie-root-port,id=root2,bus=pcie.0,chassis=2 \ -netdev tap,script=/root/bin/bridge.sh,downscript=no,id=hostnet1,vhost=on \ -device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:6f:55:cc,bus=root2,primary=hostdev0 \ -device vfio-pci,host=5e:00.2,id=hostdev0,bus=root1,standby=net1 \ /root/rhel-guest-image-8.0-1781.x86_64.qcow2 I'm grateful for any remarks or ideas! Thanks! regards, Jens Sameeh Jubran (2): qdev/qbus: Add hidden device support net/virtio: add failover support hw/core/qdev.c | 27 ++++++++++ hw/net/virtio-net.c | 95 ++++++++++++++++++++++++++++++++++ hw/pci/pci.c | 1 + include/hw/pci/pci.h | 2 + include/hw/qdev-core.h | 8 +++ include/hw/virtio/virtio-net.h | 7 +++ qdev-monitor.c | 48 +++++++++++++++-- vl.c | 7 ++- 8 files changed, 189 insertions(+), 6 deletions(-) -- 2.20.1