Re: [RFC v3] VFIO Migration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Stefan Hajnoczi (stefanha@redhat.com) wrote:
> On Wed, Nov 11, 2020 at 04:18:34PM +0000, Thanos Makatos wrote:
> > 
> > > 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?
> 
> The --print-migration-info-json command-line option described below must
> be implemented by the vfio-user device emulation program. Similarly,
> VFIO/mdev devices must provide the migration/ sysfs group.
> 
> If the device implementation does not expose these standard interfaces
> then management tools can still attempt to migrate them, but there is no
> migration compatibility check or algorithm for setting up the
> destination device. In other words, it will only succeed with some luck
> or by hardcoding knowledge of the specific device implementation into
> the management tool.
> 
> > 
> > > 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.
> 
> Yes, if they are migration compatible (they support the same device
> model and migration parameters) then migration is possible. I'll make
> this clear in the next revision.
> 
> Note VFIO migration is currently only working for mdev devices. Alex
> Williamson mentioned that it could be extended to core VFIO PCI devices
> (without mdev) in the future.
> 
> > > 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?
> 
> Yes, it's definitely possible. There needs to be agreement on the RPC
> mechanism. QEMU implements QMP, SPDK has something similar but
> different, gRPC/Protobuf is popular, and D-Bus is another alternative. I
> asked about RPC mechanisms on the muser Slack instance to see if there
> was consensus but it seems to be a bit early for that.
> 
> Perhaps the most realistic option will be to define bindings to several
> RPC mechanisms. That way everyone can use their preferred RPC mechanism,
> at the cost of requiring management tools to support more than one
> (which some already do, e.g. libvirt uses XDR itself but also implements
> QEMU's QMP).
> 
> > > 
> > > 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?
> 
> Yes, it needs to be stable in the sense that you can invoke the program
> with --print-migration-info-json and then expect launching the program
> to succeed with migration parameters that are valid according to the
> JSON.
> 
> Running the same device emulation binary on different hosts can produce
> different JSON. This is because the binary may rely on host hardware
> resources or features (e.g. does this host have GPUs available?).
> 
> It gets trickier when considering host reboots. I think the JSON can
> change between reboots. However, the management tools may cache the JSON
> so there needs to be a rule about when to refresh it.

libvirt does something similar for QEMU's current capabilities; it
normally works fine; very occasionally you have to flush the cache
though if you do something surprising which causes it to change
capabilities.

Dave

> Regarding additional command-line arguments, they can affect the JSON
> output. For example, they could include the connection details to an
> iSCSI LUN and affect the block size migration parameter. This leads to
> the same issue - can they be cached by the management tool? The answer
> is the same - stability is needed in the short-term to avoid unexpected
> failures when launching the program, but over the longer term we should
> allow JSON changes.
> 
> Thanks for raising these points. I'll add details to the next revision.
> 
> Stefan


-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK