About improving devices and renaming migration stream...

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hello Dave,

==[background]==

I've been doing this pflash rework:

* Add abstract TYPE_NOR_FLASH

  - qdev type
  - blockdev backend
  - manage bank/sector,
  - manage timer for erase/write delays
  - can be used by I2C/SPI NOR flash too

* Add abstract TYPE_PARALLEL_NOR_FLASH

  - mostly SysBusDevice bindings

* Add TYPE_COMMON_FLASH_MEMORY_INTERFACE

  - common CFI code
  - abstracts CFI methods

* Refactor TYPE_PFLASH_CFI02
  -> TYPE_PARALLEL_NOR_CFI02_FLASH

  - Inherit TYPE_NOR_FLASH -> TYPE_PARALLEL_NOR_FLASH
  - Implements TYPE_COMMON_FLASH_MEMORY_INTERFACE


I kept the harder for the end... (maybe bad idea):
  - migration
  - CFI01


==[problems]==


1/ Can I change the name of a migration stream?

I want to add migration from old TYPE_PFLASH_CFI02 to
TYPE_PARALLEL_NOR_CFI02_FLASH, and deprecate TYPE_PFLASH_CFI02.

From docs/devel/migration.rst:

  "The ``ID string`` is normally unique, having been
   formed from a bus name and device address, PCI devices
   and storage devices hung off PCI controllers fit this
   pattern well.  Some devices are fixed single instances
   (e.g. "pc-ram").
   Others (especially either older devices or system devices
   which for some reason don't have a bus concept) make use
   of the ``instance id`` for otherwise identically named
   devices."

I started TYPE_PARALLEL_NOR_CFI02_FLASH from a clean design,
I'm not sure I can redo everything keeping the same type name.

Is it possible to use massaging functions to migrate from
TYPE_A (version y) to TYPE_B (version x)?


2/ Do I need to care about backward compatibility?

I understand we want to migrate from old -> new QEMU version.

From docs/devel/migration.rst:

  "In general QEMU tries to maintain forward migration
   compatibility (i.e. migrating from QEMU n->n+1) and
   there are users who benefit from backward
   compatibility as well."

When is it important to migrate from new -> old?
Can this be a blocker?
Am I doomed to keep the old device forever?

Is this what are versioned machines for?
I.e. adding machine-v6 that start with the newer devices,
breaking backward compat.

Thanks,

Phil.