Features/Migration/Visitor

OVERVIEW

This patch series implements a QEMUFile Visitor class that's intended to abstract away direct calls to qemu_put_*/qemu_get_* for save/load functions. Currently this is done by always creating a QEMUFileInputVisitor/QEMUFileOutputVisitor pair with each call to qemu_fopen_ops() and maintaining a QEMUFile->I/O Visitor mapping. save/load functions that are to be converted would them simply use lookup routines to get a Visitor based on their QEMUFile arguments. Once these save/load functions are all converted, we can then change the interfaces in bulk and switch over to passing in the Visitor directly.

This series also converts all of vmstate over to using Visitors, leveraging the existing vmstate hierachical structure to handle namespacing and structuring of the fields that are fed to the visitor functions, and introduces trace statements for qemu_put_*/qemu_get_* and visitor API calls to aid in testing.

Migration compatibility is not affected by these changes, as the qemu_put_*/qemu_get_* calls have a trivial 1-to-1 mapping with the visit_type_* calls, and the added field names/structure are not sent out over the wire when using the QEMUFile visitors, but serve simply as placeholders for future functionality.

PLANS

With these patches in place non-vmstate save/load functions can be converted over to using Visitors incrementally.

Short term (1.1 timeframe), the goal is to implement a new migration protocol that is self-describing, such that incompatibilities or migration errors can be more easilly detected. Currently, a simple change in data types for a particular device can introduce subtle bugs that won't be detected by the target, since the target interprets the data according to it's own expectation of what those data types are. Currently the plan is to achieve this by using ASN.1 BER in place of QEMUFile via a new BERVisitor.

Also planned is using the visitor interface to aid in things like device introspection (for both vmstate and non-vmstate devices), likely via the existing QMPOutputVisitor, as well as debugging/testing migration via a well structured "PrettyPrintVisitor" type of visitor that also retains ordering, field names, and type information (unlike with QMP/JSON or ASN.1).

It's not completely clear at this point how the latter will fall into place with other plans such as QOM that would provide similar capabilities, but since device serialization will likely be done via automatically-generated visitor patterns, this conversion should at least serve as an incremental step toward that transition.