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(-)
