RE: [RFC v3] VFIO Migration

[Thanos Makatos]

> VFIO Migration
> ==============
> This document describes how to ensure migration compatibility for VFIO
> devices,
> including mdev and vfio-user devices.

Is this something all VFIO/user devices will have to support? If it's not
mandatory, how can a device advertise support?

> Multiple device implementations can support the same device model. Doing
> so
> means that the device implementations can offer migration compatiblity
> because
> they support the same hardware interface, device state representation, and
> migration parameters.

Does the above mean that a passthrough function can be migrated to a vfio-user
program and vice versa? If so, then it's worth mentioning.

> More complex device emulation programs may host multiple devices. The
> interface
> for configuring these device emulation programs is not standardized.
> Therefore,
> migrating these devices is beyond the scope of this document.

Most likely a device emulation program hosting multile devices would allow
some form of communication for control purposes (e.g. SPDK implements a JSON-RPC
server). So maybe it's possible to define interacting with such programs in
this document?

> 
> The migration information JSON is printed to standard output by a vfio-user
> device emulation program as follows:
> 
> .. code:: bash
> 
>   $ my-device --print-migration-info-json
> 
> The device is instantiated by launching the destination process with the
> migration parameter list from the source:

Must 'my-device --print-migration-info-json' always generate the same migration
information JSON? If so, then what if the output generated by
'my-device --print-migration-info-json' depends on additional arguments passed
to 'my-device' when it was originally started?