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, especially when migrating zeropages or skipping migration of some pages.
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. When migrating the zeropage, we might not end up allocating actual backend memory. 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. For postcopy to work with prealloc=on, we need a matching "requested-size" on source and destination, meaning we have to start QEMU on the destination with the current "requested-size" on the source. Only that way, we can try temporarily allocating the "requested-size" to see if there is a fundamental issue. If we detect a mismatch, we don't start postcopy. Cc: Juan Quintela <quint...@redhat.com> Cc: Dr. David Alan Gilbert <dgilb...@redhat.com> Cc: Michael S. Tsirkin <m...@redhat.com> Cc: Michal Privoznik <mpriv...@redhat.com> David Hildenbrand (2): virtio-mem: Warn if a memory backend with "prealloc=on" is used virtio-mem: Handle preallocation with migration hw/virtio/virtio-mem.c | 143 +++++++++++++++++++++++++++++++++ include/hw/virtio/virtio-mem.h | 6 ++ 2 files changed, 149 insertions(+) -- 2.34.1
