While playing with migration of virtio-mem with an ordinary file backing, I realized that migration and prealloc doesn't currently work as expected for virtio-mem. Further, Jing Qi reported that setup issues (insufficient huge pages on the destination) result in QEMU getting killed with SIGBUS instead of failing gracefully.
In contrast to ordinary memory backend preallocation, virtio-mem preallocates memory before plugging blocks to the guest. Consequently, when migrating we are not actually preallocating on the destination but "only" migrate pages. Fix that be migrating the bitmap early, before any RAM content, and use that information to preallocate memory early, before migrating any RAM. Postcopy needs some extra care, and I realized that prealloc+postcopy is shaky in general. Let's at least try to mimic what ordinary prealloc+postcopy does: temporarily allocate the memory, discard it, and cross fingers that we'll still have sufficient memory when postcopy actually tries placing pages. Cc: Dr. David Alan Gilbert <dgilb...@redhat.com> Cc: Juan Quintela <quint...@redhat.com> Cc: Peter Xu <pet...@redhat.com> Cc: Michael S. Tsirkin <m...@redhat.com> Cc: Michal Privoznik <mpriv...@redhat.com> v2 -> v3: - New approach/rewrite, drop RB and TB of last patch v1 -> v2: - Added RBs and Tested-bys - "virtio-mem: Fail if a memory backend with "prealloc=on" is specified" -- Fail instead of warn -- Adjust subject/description David Hildenbrand (6): migration: Allow immutable device state to be migrated early (i.e., before RAM) migration/vmstate: Introduce VMSTATE_WITH_TMP_TEST() and VMSTATE_BITMAP_TEST() migration: Factor out checks for advised and listening incomming postcopy virtio-mem: Fail if a memory backend with "prealloc=on" is specified virtio-mem: Migrate bitmap, size and sanity checks early virtio-mem: Proper support for preallocation with migration hw/core/machine.c | 4 +- hw/virtio/virtio-mem.c | 154 ++++++++++++++++++++++++++++++++- include/hw/virtio/virtio-mem.h | 8 ++ include/migration/misc.h | 6 +- include/migration/vmstate.h | 19 +++- migration/migration.c | 27 ++++++ migration/migration.h | 4 + migration/ram.c | 16 +--- migration/savevm.c | 104 ++++++++++++++++------ migration/savevm.h | 1 + 10 files changed, 298 insertions(+), 45 deletions(-) -- 2.38.1