Re: [Qemu-devel] [RFC PATCH 3/4] savevm: define new unambiguous migration format

[Anthony Liguori <anthony@codemonkey.ws>]

On 06/30/2011 10:46 AM, Paolo Bonzini wrote:
> With the current migration format, VMS_STRUCTs with subsections
> are ambiguous.  The protocol cannot tell whether a 0x5 byte after
> the VMS_STRUCT is a subsection or part of the parent data stream.
> In the past QEMU assumed it was always a part of a subsection; after
> commit eb60260 (savevm: fix corruption in vmstate_subsection_load().,
> 2011-02-03) the choice depends on whether the VMS_STRUCT has subsections
> defined.
>
> Unfortunately, this means that if a destination has no subsections
> defined for the struct, it will happily read subsection data into
> its own fields.  And if you are "lucky" enough to stumble on a
> zero byte at the right time, it will be interpreted as QEMU_VM_EOF
> and migration will be interrupted.
>
> There is no way out of this except defining an incompatible
> migration protocol with a sentinel at the end of embedded structs.
> Of course, this is restricted to new machine models.
>
> Signed-off-by: Paolo Bonzini<pbonzini@redhat.com>
> ---
>   hw/pc_piix.c |    6 ++++++
>   savevm.c     |   27 +++++++++++++++++++--------
>   2 files changed, 25 insertions(+), 8 deletions(-)
>
> diff --git a/hw/pc_piix.c b/hw/pc_piix.c
> index 18cc942..d8d629c 100644
> --- a/hw/pc_piix.c
> +++ b/hw/pc_piix.c
> @@ -271,6 +271,7 @@ static QEMUMachine pc_machine_v0_14 = {
>       .desc = "Standard PC",
>       .init = pc_init_pci,
>       .max_cpus = 255,
> +    .migration_format = 3,
>   };

Please introduce a macro so this code is readable.

We have other machines that support migration in other archs too.

Regards,

Anthony Liguori