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[Qemu-devel] [RFC PATCH 0/2] implement the failover feature for assigned


From: Jens Freimann
Subject: [Qemu-devel] [RFC PATCH 0/2] implement the failover feature for assigned network devices
Date: Fri, 22 Mar 2019 14:44:45 +0100

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




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