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

[Jens Freimann]

On Fri, Apr 05, 2019 at 09:56:29AM +0100, Dr. David Alan Gilbert wrote:
> * Jens Freimann (address@hidden) wrote:
[...]
> > > 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.
>
> This sounds 'fun' - bonus cases are things like what happens if the
> guest gets rebooted somewhere during the process or if it's currently
> sitting in the bios/grub/etc

Yeah, I have to think about this...

> > > 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).
>
> Are they really that precious? Personally it's not something I'd worry
> about.
>
> > > 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.
>
> Why not just let this happen at the libvirt level; then you do the
> hotunplug etc before you actually tell qemu anything about starting a
> migration?

Yes...the goal was to see if we can contain changes to QEMU (to keep
it simple, although it seems that covering all error cases won't be that simple 
:).
But I don't see a mechanism to trigger the unplug at the right moment
yet.  So yes, maybe there's no way around involving libvirt at least
for this part...

> > > 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 \
>
> Yes, that's a bit grim; it's circular dependency on the 'hostdev0' and
> 'net1' id's.  cc'ing in Markus.

Dan had an idea how to avoid having to specify the id for the
virtio-net device. I'm currently looking into it, but it seems like it
should work. 


regards,
Jens