Re: VFIO Migration

[Dr. David Alan Gilbert]

* Michael S. Tsirkin (mst@redhat.com) wrote:
> 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.

So if I ingore parameters, do I get the latest, greatest config from the
device implementation I have?

> > > 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.

That's why I liked the 'version' idea; you just have to find the lowest
version in your set.

Dave

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