[Qemu-devel] [RFC 0/2] Attempt to implement the standby feature for assigned network devices

[Sameeh Jubran <sameeh@daynix.com>]

From: Sameeh Jubran <sjubran@redhat.com>

Hi all,

Background:

There has been a few attempts to implement the standby feature for vfio
assigned devices which aims to enable the migration of such devices. This
is another attempt.

The series implements an infrastructure for hiding devices from the bus
upon boot. What it does is the following:

* 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.

Disclaimers:

* I have only scratch tested this and from qemu side, it seems to be
  working.
* This is an RFC so it lacks some proper error handling in few cases
  and proper resource freeing. I wanted to get some feedback first
  before it is finalized.

Command line example:

/home/sameeh/Builds/failover/qemu/x86_64-softmmu/qemu-system-x86_64 \
-netdev tap,id=hostnet0,script=world_bridge_standalone.sh,downscript=no,ifname=cc1_71 \
-netdev tap,vhost=on,id=hostnet1,script=world_bridge_standalone.sh,downscript=no,ifname=cc1_72,queues=4 \
-device virtio-net,host_mtu=1500,netdev=hostnet1,id=cc1_72,vectors=10,mq=on,primary=cc1_71 \
-device e1000,netdev=hostnet0,id=cc1_71,standby=cc1_72 \

Migration support:

Pre migration or during setup phase of the migration we should send an
unplug request to the guest to unplug the primary device. I haven't had
the chance to implement that part yet but should do soon. Do you know
what's the best approach to do so? I wanted to have a callback to the
virtio-net device which tries to send an unplug request to the guest and
if succeeds then the migration continues. It needs to handle the case where
the migration fails and then it has to replug the primary device back.

The following terms are used as interchangeable:
standby - virtio-net
primary - vfio-device - physical device - assigned device

Please share your thoughts and suggestions,
Thanks!

Sameeh Jubran (2):
  qdev/qbus: Add hidden device support
  virtio-net: Implement VIRTIO_NET_F_STANDBY feature

 hw/core/qdev.c                 | 48 +++++++++++++++++++++++++---
 hw/net/virtio-net.c            | 25 +++++++++++++++
 hw/pci/pci.c                   |  1 +
 include/hw/pci/pci.h           |  2 ++
 include/hw/qdev-core.h         | 11 ++++++-
 include/hw/virtio/virtio-net.h |  5 +++
 qdev-monitor.c                 | 58 ++++++++++++++++++++++++++++++++--
 7 files changed, 142 insertions(+), 8 deletions(-)

-- 
2.17.0