virtio-mem: Migrate immutable properties early

The bitmap and the size are immutable while migration is active: see
virtio_mem_is_busy(). We can migrate this information early, before
migrating any actual RAM content. Further, all information we need for
sanity checks is immutable as well.

Having this information in place early will, for example, allow for
properly preallocating memory before touching these memory locations
during RAM migration: this way, we can make sure that all memory was
actually preallocated and that any user errors (e.g., insufficient
hugetlb pages) can be handled gracefully.

In contrast, usable_region_size and requested_size can theoretically
still be modified on the source while the VM is running. Keep migrating
these properties the usual, late, way.

Use a new device property to keep behavior of compat machines
unmodified.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>S
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/hw/virtio/virtio-mem.h b/include/hw/virtio/virtio-mem.h
index 7745cfc1a3..f15e561785 100644
--- a/include/hw/virtio/virtio-mem.h
+++ b/include/hw/virtio/virtio-mem.h
@@ -31,6 +31,7 @@ OBJECT_DECLARE_TYPE(VirtIOMEM, VirtIOMEMClass,
 #define VIRTIO_MEM_BLOCK_SIZE_PROP "block-size"
 #define VIRTIO_MEM_ADDR_PROP "memaddr"
 #define VIRTIO_MEM_UNPLUGGED_INACCESSIBLE_PROP "unplugged-inaccessible"
+#define VIRTIO_MEM_EARLY_MIGRATION_PROP "x-early-migration"
 #define VIRTIO_MEM_PREALLOC_PROP "prealloc"
 
 struct VirtIOMEM {
@@ -74,6 +75,13 @@ struct VirtIOMEM {
     /* whether to prealloc memory when plugging new blocks */
     bool prealloc;
 
+    /*
+     * Whether we migrate properties that are immutable while migration is
+     * active early, before state of other devices and especially, before
+     * migrating any RAM content.
+     */
+    bool early_migration;
+
     /* notifiers to notify when "size" changes */
     NotifierList size_change_notifiers;