migration/ram: Handle RAM block resizes during precopy

Resizing while migrating is dangerous and does not work as expected.
The whole migration code works on the usable_length of ram blocks and does
not expect this to change at random points in time.

In the case of precopy, the ram block size must not change on the source,
after syncing the RAM block list in ram_save_setup(), so as long as the
guest is still running on the source.

Resizing can be trigger *after* (but not during) a reset in
ACPI code by the guest
- hw/arm/virt-acpi-build.c:acpi_ram_update()
- hw/i386/acpi-build.c:acpi_ram_update()

Use the ram block notifier to get notified about resizes. Let's simply
cancel migration and indicate the reason. We'll continue running on the
source. No harm done.

Update the documentation. Postcopy will be handled separately.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210429112708.12291-5-david@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Manual merge

1 files changed, 30 insertions, 0 deletions

diff --git a/migration/ram.c b/migration/ram.c
index bb52bd97db..77922c445e 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -4096,8 +4096,38 @@ static SaveVMHandlers savevm_ram_handlers = {
     .resume_prepare = ram_resume_prepare,
 };
 
+static void ram_mig_ram_block_resized(RAMBlockNotifier *n, void *host,
+                                      size_t old_size, size_t new_size)
+{
+    ram_addr_t offset;
+    RAMBlock *rb = qemu_ram_block_from_host(host, false, &offset);
+    Error *err = NULL;
+
+    if (ramblock_is_ignored(rb)) {
+        return;
+    }
+
+    if (!migration_is_idle()) {
+        /*
+         * Precopy code on the source cannot deal with the size of RAM blocks
+         * changing at random points in time - especially after sending the
+         * RAM block sizes in the migration stream, they must no longer change.
+         * Abort and indicate a proper reason.
+         */
+        error_setg(&err, "RAM block '%s' resized during precopy.", rb->idstr);
+        migrate_set_error(migrate_get_current(), err);
+        error_free(err);
+        migration_cancel();
+    }
+}
+
+static RAMBlockNotifier ram_mig_ram_notifier = {
+    .ram_block_resized = ram_mig_ram_block_resized,
+};
+
 void ram_mig_init(void)
 {
     qemu_mutex_init(&XBZRLE.lock);
     register_savevm_live("ram", 0, 4, &savevm_ram_handlers, &ram_state);
+    ram_block_notifier_add(&ram_mig_ram_notifier);
 }