Re: VFIO Migration

[Michael S. Tsirkin]

On Thu, Nov 05, 2020 at 12:13:24PM +0000, Dr. David Alan Gilbert wrote:
> * Stefan Hajnoczi (stefanha@redhat.com) wrote:
> > On Wed, Nov 04, 2020 at 05:32:02PM +0000, Dr. David Alan Gilbert wrote:
> > > * Stefan Hajnoczi (stefanha@redhat.com) wrote:
> > > > Michael replied in another sub-thread wondering if versions are really
> > > > necessary since tools do the migration checks. Let's try dropping
> > > > versions to simplify things. We can bring them back if needed later.
> > > 
> > > What does a user facing tool do?  If I say I want one of these NICs
> > > and I'm on the latest QEMU machine type, who sets all these parameters?
> > 
> > The machine type is orthogonal since QEMU doesn't know about every
> > possible VFIO device. The device is like a PCI adapter that is added to
> > a physical machine aftermarket, it's not part of the base machine's
> > specs.
> 
> OK, but ignoring migration, I think the same problem holds; if I'm a
> tool creating one of these VMs, and I plug this device in, what do I do
> with all it's configuration parameters?  I'd assume most of the time
> that they don't know about or dont care about most of the parameters,
> they just want the sane defaults unless told otherwise.

I think that if you ignore migration then you can ignore parameters.

> > The migration tool queries the parameters from the source device.
> > VFIO/mdev will provide sysfs attrs. For vfio-user I'm not sure whether
> > to print the parameters during device instantiation, require a
> > VFIO-compatible FUSE directory, or to use a query-migration-params RPC
> > command.
> 
> But on VM creation we have to answer the question of what config do we
> want; so for example lets say I'm creating a new VM in my cluster,
> but I want to be sure that later I can migrate it.  I can read the 
> config off one of the other machines;  can I just use that even if my
> new machine has a later device implementation?
> 
> Dave

I don't think so - we need a tool that can query a set of machines and then
produce a safe configuration.

The same problem exists with vhost as well it will just
explode exponentially with lots more devices and backends ...
Talked about it a bit in my kvm forum preso ...

> > Let's discuss this more when the next revision of the document is sent
> > out, because it modifies the approach so that migration parameters are
> > logically separate from device configuration parameters. That changes
> > things a bit.
> > 
> > Stefan
> 
> 
> -- 
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK