Features/Migration/Next

Today

Today we only support generating the latest serialization of devices. To increase the probability of the latest version working on older versions of QEMU, we strategically omit fields that we know can safely be omitted with older versions (subsections). More than likely, migrating new to old won't work.

Migrating old to new is more likely to work. We version each section in order to be able to identify when we're dealing with old.

But all of this logic lives in one of two forms. Either as a savevm/loadvm callback that takes a QEMUFile and writes byte serialization to the stream in an open way (usually big endian) or encoded declaratively in a VMState section.

What we need

We need to decompose migration into three different problems:

1. serializing device state
2. transforming the device model in order to satisfy forwards and backwards compatibility
3. encoding the serialized device model on the wire.

We also need a way to future proof ourselves.

What we can do

Future Proofing

Add migration capabilities to future proof ourselves. I think the simplest way this would work is to have a 'query-migration-capabilities' command that returned a bitmask of supported migration features. I think we also introduce a 'set-migration-capabilities' command that can mask some of the supported features.

A management tool would query-migration features on the source and destination, take the intersection of the two masks, and set that mask on both the source and destination.

Lack of support for these commands indicates a mask of zero which is the protocol we offer today.

Using Visitors

Switch to a visitor model to serialize device state. This involves converting any occurance of:

qemu_put_be32(f, port->guest_connected);

To:

visit_type_u32(v, "guest_connected", &port->guest_connected, &local_err);

It's 100% mechanical and makes absolutely no logic change. It works equally well with legacy and VMstate migration handlers.

QEMUFile Visitor

Add a Visitor class that operates on QEMUFile.

At this state, we can migrate to data structures. That means we can migrate to QEMUFile, QObjects, or JSON. We could change the protocol at this stage to something that was still binary but had section sizes and things of that nature.

But we shouldn't stop here.

Device Model Transformation

Compatibility logic should be extracted from the savevm functions and VMstate functions into separate functions that take a data structure. Basically, we want to have something roughly equivalent to:

QObject *e1000_migration_compatibility(QObject *src, int src_version, int dst_version);

We can have lots of helpers that reuse the VMstate declarative stuff to do this but this should be registered independent of the main serialization handler.

This moves us to a model where we always generate the latest serialization format, and then have specific ways to convert to older mechanisms. It allows us to do very big backwards compatibility steps like convert the state of one device into two separate devices (because we're just dealing with in-memory data structures).

It's this step that lets us truly support compatibility with migration. The good news is, it doesn't have to be all or nothing. Since we always already generate the latest serialization format, the existing code only deals with migrating older versions to the latest which is something that isn't all that important.

So if we did this in 1.0, we could have a single function that converted the 1.0 device model to 1.1 and vice versa, and we'd be fine. We wouldn't have to touch 200 devices to do this.

Next Gen Transport

Once we're here, we can implement the next 5-year format. That could be ASN.1 and be bidirectional or whatever makes the most sense. We could support 50 formats if we wanted to. As long as the transport is distinct from the serialization and compat routines, it really doesn't matter.