Re: [PATCH v6 07/18] cpu: Rename CPUClass vmsd -> legacy_vmsd

[Richard Henderson <richard.henderson@linaro.org>]

On 4/22/21 12:38 PM, Philippe Mathieu-Daudé wrote:
> 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 b170fce3dd0 ("cpu: Register VMStateDescription
> through CPUState") for historic background.
> 
> [*]https://www.mail-archive.com/qemu-devel@nongnu.org/msg800849.html
> 
> Cc: Peter Maydell<peter.maydell@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé<f4bug@amsat.org>
> ---
>   include/hw/core/cpu.h           |  5 +++--
>   cpu.c                           | 12 ++++++------
>   target/arm/cpu.c                |  2 +-
>   target/avr/cpu.c                |  2 +-
>   target/i386/cpu.c               |  2 +-
>   target/lm32/cpu.c               |  2 +-
>   target/mips/cpu.c               |  2 +-
>   target/moxie/cpu.c              |  2 +-
>   target/riscv/cpu.c              |  2 +-
>   target/s390x/cpu.c              |  2 +-
>   target/sparc/cpu.c              |  2 +-
>   target/ppc/translate_init.c.inc |  2 +-
>   12 files changed, 19 insertions(+), 18 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~