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)
Changes since last RFC: - work around circular dependency of commandline options. Just add failover=on to the virtio-net standby options and reference it from primary (vfio-pci) device with standby=<id> - add patch 3/4 to allow migration of vfio-pci device when it is part of a failover pair, still disallow for all other devices - add patch 4/4 to allow unplug of device during migrationm, make an exception for failover primary devices. I'd like feedback on how to solve this more elegant. I added a boolean to DeviceState, have it default to false for all devices except for primary devices. - not tested yet with surprise removal - I don't expect this to go in as it is, still needs more testing but I'd like to get feedback on above mentioned changes. 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. * In the second patch the virtio-net uses the API to defer adding the vfio device until the VIRTIO_NET_F_STANDBY feature is acked. Previous discussion: RFC v1 https://patchwork.ozlabs.org/cover/989098/ RFC v2 https://www.mail-archive.com/qemu-devel@nongnu.org/msg606906.html 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. 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,failover=on \ /root/rhel-guest-image-8.0-1781.x86_64.qcow2 Then the primary device can be hotplugged via (qemu) device_add vfio-pci,host=5e:00.2,id=hostdev0,bus=root1,standby=net1 I'm grateful for any remarks or ideas! Thanks! Jens Freimann (4): migration: allow unplug during migration for failover devices qdev/qbus: Add hidden device support net/virtio: add failover support vfio: unplug failover primary device before migration hw/core/qdev.c | 20 ++++++ hw/net/virtio-net.c | 117 +++++++++++++++++++++++++++++++++ hw/vfio/pci.c | 25 ++++++- hw/vfio/pci.h | 2 + include/hw/qdev-core.h | 10 +++ include/hw/virtio/virtio-net.h | 12 ++++ qdev-monitor.c | 43 ++++++++++-- vl.c | 6 +- 8 files changed, 228 insertions(+), 7 deletions(-) -- 2.21.0