[Qemu-devel] [PATCH for-2.8 0/3] spapr: fix breakage of memory unplug after migration

[Michael Roth]

These patches are based on David's ppc-for-2.8 tree, and are also
available from:

  https://github.com/mdroth/qemu/commits/spapr-cas-migration

Currently, memory hotplugged to a pseries guest cannot be removed after
the guest has been migrated. This is due to 2 issues:

1) The coldplugged state of memory on the target side is one where the
   corresponding DRC's allocation state is:

     allocation_state == unallocated,
     awaiting_allocation == true,

   When the guest attempts to unplug memory on the target side, it first
   checks that allocation_state == allocated. If we fix this, the guest
   can successfully notify QEMU of completion on it's end, but then the
   DRC code sees that awaiting_allocation == true, so it defers the
   finalizing of the LMB and corresponding DIMM since it assumes that
   the DIMM must have been previously allocated before it can be removed.

   To address this, we pull in patches 1-2 from Jian Jun's DRC migration
   series:

     https://lists.gnu.org/archive/html/qemu-ppc/2016-10/msg00048.html

   with some minor changes relating to prior review comments, and
   the addition of migrating the DRC's awaiting_allocation value, which
   wasn't part of the original patch. This doesn't address the full scope
   of the issues Jian Jun was looking at (involving synchronizing state
   when migration occurs during fairly small race windows), just this
   particular case, which is more user visible since the time window is
   indefinite.

2) The ability to unplug memory is gated on the QEMU side by a check as
   to whether or not support for newer-style hotplug events was negotiated
   via CAS during boot. The check is performed by checking the corresponding
   entry in the sPAPROptionVector structure. However, since this value isn't
   migrated currently, we are unable to unplug until after the guest reboots.

   We address that here by adding migration support for sPAPROptionVectors,
   and including the CAS-negotiated vector as part of the migration stream
   for any cases where we advertise newer-style hotplug event support to
   the guest.

David,

These fixes ended up going out much later than planned. I'm not sure
if you're planning another pull for 2.8 or not, and realize there are
some patches here not specifically pseries-related so it's
understandable if we opt to pursue these for 2.9/2.8.1 instead. But if
possible I'm hoping to get these in so that the memory unplug
support is fully functional for 2.8.


 hw/core/qdev.c              |  6 +++++-
 hw/ppc/spapr.c              | 68 
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 hw/ppc/spapr_drc.c          | 70 
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 hw/ppc/spapr_ovec.c         | 12 ++++++++++++
 hw/ppc/spapr_pci.c          | 22 ++++++++++++++++++++++
 include/hw/ppc/spapr_drc.h  |  9 +++++++++
 include/hw/ppc/spapr_ovec.h |  4 ++++
 include/hw/qdev-core.h      |  9 +++++++++
 migration/savevm.c          |  4 ++--
 9 files changed, 201 insertions(+), 3 deletions(-)