Re: [Qemu-devel] [PATCH v2 0.15 0/4] Fix subsection ambiguity in the migration format

[Anthony Liguori <anthony@codemonkey.ws>]

On 07/29/2011 10:33 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 with half-loaded state.
>
> There is no way out of this except defining an incompatible
> migration protocol.  Not-so-long-term we should really try to define
> one that is not a joke, but the bug is serious so we need a solution
> for 0.15.  A sentinel at the end of embedded structs does remove the
> ambiguity.

I've thought about this very carefully now.  I just don't feel 
comfortable making a protocol change in an rc window for a series that 
hasn't spent any time in master.

This issue needs to be fixed for 0.15, but there's a simpler solution as 
we currently only have two uses of subsections in the tree today.  I'll 
send out a patch that bumps those two migration states to a new version 
and eliminates the subsection usage entirely.

If we can agree on that for 0.15, I'm happy to take this series into 
master but we should also consider other possibilities too for fixing 
the problem.

Regards,

Anthony Liguori

>
> Of course, this can be restricted to new machine models, and this
> is what the patch series does.  (And note that only patch 3 is specific
> to the short-term solution, everything else is entirely generic).
>
> I am still proposing this for 0.15.  Tested new on new, 0.14 on new
> pc-0.14, new pc-0.14 on 0.14; also for v1 the same combinations on RHEL.
>
> v1->v2:
>          added qemu_current_migration_format() and
>          QEMU_VM_FILE_VERSION_0_14.
>
> Paolo Bonzini (4):
>    add support for machine models to specify their migration format
>    add pc-0.14 machine
>    savevm: define new unambiguous migration format
>    Partially revert "savevm: fix corruption in
>      vmstate_subsection_load()."
>
>   cpu-common.h  |    3 ---
>   hw/boards.h   |    4 ++++
>   hw/pc_piix.c  |   15 ++++++++++++++-
>   qemu-common.h |    3 +++
>   savevm.c      |   46 ++++++++++++++++++++++++++++++++--------------
>   5 files changed, 53 insertions(+), 18 deletions(-)
>