On Fri, 17 May 2019 14:58:16 +0200 Jens Freimann <jfreim...@redhat.com> wrote:
> 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 Is this standby= option only valid for Network/Ethernet class code devices? If so, perhaps vfio-pci code should reject the option on any non-ethernet devices. The option is also non-intuitive for users, only through examples like above can we see it relates to the id of the secondary device. Could we instead name it something like "standby_net_failover_pair_id="? Also, this feature requires matching MAC addresses per the description, where is that done? Is it the user's responsibility to set the MAC on the host device prior to the device_add? If so, is this actually not only specific to ethernet devices, but ethernet VFs? Finally, please copy me on code touching vfio. Thanks, Alex
