Quoting Peter Maydell [*]:

  There are two ways to handle migration for
  a CPU object:

  (1) like any other device, so it has a dc->vmsd that covers
  migration for the whole object. As usual for objects that are a
  subclass of a parent that has state, the first entry in the
  VMStateDescription field list is VMSTATE_CPU(), which migrates
  the cpu_common fields, followed by whatever the CPU's own migration
  fields are.

  (2) a backwards-compatible mechanism for CPUs that were
  originally migrated using manual "write fields to the migration
  stream structures". The on-the-wire migration format
  for those is based on the 'env' pointer (which isn't a QOM object),
  and the cpu_common part of the migration data is elsewhere.

  cpu_exec_realizefn() handles both possibilities:

  * for type 1, dc->vmsd is set and cc->vmsd is not,
    so cpu_exec_realizefn() does nothing, and the standard
    "register dc->vmsd for a device" code does everything needed

  * for type 2, dc->vmsd is NULL and so we register the
    vmstate_cpu_common directly to handle the cpu-common fields,
    and the cc->vmsd to handle the per-CPU stuff

  You can't change a CPU from one type to the other without breaking
  migration compatibility, which is why some guest architectures
  are stuck on the cc->vmsd form. New targets should use dc->vmsd.

To avoid new targets to start using type (2), rename cc->vmsd as
cc->legacy_vmsd. The correct field to implement is dc->vmsd (the
DeviceClass one).

See also commit b170fce3 ("cpu: Register VMStateDescription
through CPUState") for historic background.

[*] https://www.mail-archive.com/qemu-devel@nongnu.org/msg800849.html



Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210517105140.1062037-13-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>