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

[Jens Freimann <jfreimann@redhat.com>]

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