Re: [Qemu-devel] [PATCH 0/4] add failover feature for assigned network devices

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, May 17, 2019 at 02:58:16PM +0200, Jens Freimann 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

This command line syntax looks much saner now that the circular dep is
gone. I think this approach is now viable to use from libvirt's POV.

Regards,
Daniel
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