Re: [PATCH v8 16/19] qapi: introduce backend-transfer migration parameter

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

On Thu, Oct 16, 2025 at 02:40:58PM -0400, Peter Xu wrote:
> On Thu, Oct 16, 2025 at 12:23:35PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > On 16.10.25 11:32, Daniel P. Berrangé wrote:
> > > On Thu, Oct 16, 2025 at 12:02:45AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > > > 1. Remote migration: we can't reuse backends (files, sockets, host devices), as
> > > > we are moving to another host. So, we don't enable "backend-transfer". We don't
> > > > transfer the backend, we have to initialize new backend on another host.
> > > > 
> > > > 2. Local migration to update QEMU, with minimal freeze-time and minimal
> > > > extra actions: use "backend-transfer", exactly to keep the backends
> > > > (vhost-user-server, TAP device in kernel, in-kernel vfio device state, etc)
> > > > as is.
> > > > 
> > > > 3. Local migration, but we want to reconfigure some backend, or switch
> > > > to another backend. We disable "backend-transfer" for one device.
> > > 
> > > This implies that you're changing 'backend-transfer' against the
> > > device at time of each migration.
> > > 
> > > This takes us back to the situation we've had historically where the
> > > behaviour of migration depends on global properties the mgmt app has
> > > set prior to the 'migrate' command being run. We've just tried to get
> > > away from that model by passing everything as parameters to the
> > > migrate command, so I'm loathe to see us invent a new way to have
> > > global state properties changing migration behaviour.
> > > 
> > > This 'backend-transfer' device property is not really a device property,
> > > it is an indirect parameter to the 'migrate' command.
> 
> I was not seeing it like that.
> 
> I was treating per-device parameter to be a flag showing whether the device
> is capable of passing over FDs, which is more like a device attribute.
> 
> Those things (after set by machine type) should never change, and the only
> thing to be changed is the global "backend-transfer" boolean that can be
> set in the "migrate" QMP command, and should be decided by the admin when
> one wants to initiate the migration process.
> 
> > > 
> > > Ergo, if we need the ability to selectively migrate the backend state
> > > of individal devices, then instead of a property on the device, we
> > > should pass a list of device IDs as a parameter to the migrate
> > > command in QMP.
> 
> I doubt whether we would really need that in reality.
> 
> Likely the admin should only worry about whether setting the global
> "backend-transfer", the admin may not even need to know which device, and
> how many devices, will be beneficial to this feature enabled.
> 
> It just says, "we're doing local migration and via unix sockets, so
> whatever devices can try to reuse their backends if possible".

An individual device can only use backend transfer if both the old and
new QEMU agree that it can be done. At the time we start the origin
QEMU we know which set of devices are capable of doing an outgoing
backend transfer, but we don't know what set of devices are capable
of doing an incoming backend transfer.

If we don't have a per-device toggle at time of migration, then we
have to assume that the target QEMU can always support at least the
same set of incoming backends as the src QEMU outgoing backend. This
feels like a potentially risky assumption.

Another scenario is where you are doing a localhost migration as a
mechanism to let you change a device backend. In that case you'll
want to do a backend transfer of all devices, except the one that
you want to change.

With regards,
Daniel
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