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

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

On Thu, Oct 16, 2025 at 03:39:03PM -0400, Peter Xu wrote:
> On Thu, Oct 16, 2025 at 08:19:37PM +0100, Daniel P. Berrangé wrote:
> > On Thu, Oct 16, 2025 at 07:51:42PM +0100, Daniel P. Berrangé wrote:
> > > 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.
> > 
> > Whether a backend is technically capable of transfer shouldn't require a
> > user specified property - there should be an internal API to query whether
> > the current backend configuration is transferrable or not, based on the
> > code implementation. Allowing a mgmt app to specify this can only lead
> > to mistakes, because they don't know the internal constraints of the
> > implementation.
> > 
> > The mgmt app should only be concerned with whether they want to transfer
> > a backend or not which is a time-of-use decision rather than launch time
> > decision.
> 
> IMHO the per-device property, when available, should always mean it fully
> support the feature, when it is turned ON.

That can't be expressed in a property in the device.

Consider the virtio-net device.  The backend transfer is only
possible of the virtio-net is associated with a netdev using
the vhost-user backend, and the vhost-user backend must be
using a chardev with a socket backend, and the socket backend
must not have TLS or websockets enabled.

Migratability of the backend requires an API against the
NetClientInfo object, which will in turn require calling
out to an API against the Chardv object.


With regards,
Daniel
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