migration/ram: Introduce 'mapped-ram' migration capability

Add a new migration capability 'mapped-ram'.

The core of the feature is to ensure that RAM pages are mapped
directly to offsets in the resulting migration file instead of being
streamed at arbitrary points.

The reasons why we'd want such behavior are:

 - The resulting file will have a bounded size, since pages which are
   dirtied multiple times will always go to a fixed location in the
   file, rather than constantly being added to a sequential
   stream. This eliminates cases where a VM with, say, 1G of RAM can
   result in a migration file that's 10s of GBs, provided that the
   workload constantly redirties memory.

 - It paves the way to implement O_DIRECT-enabled save/restore of the
   migration stream as the pages are ensured to be written at aligned
   offsets.

 - It allows the usage of multifd so we can write RAM pages to the
   migration file in parallel.

For now, enabling the capability has no effect. The next couple of
patches implement the core functionality.

Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240229153017.2221-8-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

1 files changed, 34 insertions, 0 deletions

diff --git a/migration/options.c b/migration/options.c
index 1cd3cc7c33..5df89820fd 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -204,6 +204,7 @@ Property migration_properties[] = {
     DEFINE_PROP_MIG_CAP("x-switchover-ack",
                         MIGRATION_CAPABILITY_SWITCHOVER_ACK),
     DEFINE_PROP_MIG_CAP("x-dirty-limit", MIGRATION_CAPABILITY_DIRTY_LIMIT),
+    DEFINE_PROP_MIG_CAP("mapped-ram", MIGRATION_CAPABILITY_MAPPED_RAM),
     DEFINE_PROP_END_OF_LIST(),
 };
 
@@ -263,6 +264,13 @@ bool migrate_events(void)
     return s->capabilities[MIGRATION_CAPABILITY_EVENTS];
 }
 
+bool migrate_mapped_ram(void)
+{
+    MigrationState *s = migrate_get_current();
+
+    return s->capabilities[MIGRATION_CAPABILITY_MAPPED_RAM];
+}
+
 bool migrate_ignore_shared(void)
 {
     MigrationState *s = migrate_get_current();
@@ -645,6 +653,32 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp)
         }
     }
 
+    if (new_caps[MIGRATION_CAPABILITY_MAPPED_RAM]) {
+        if (new_caps[MIGRATION_CAPABILITY_MULTIFD]) {
+            error_setg(errp,
+                       "Mapped-ram migration is incompatible with multifd");
+            return false;
+        }
+
+        if (new_caps[MIGRATION_CAPABILITY_XBZRLE]) {
+            error_setg(errp,
+                       "Mapped-ram migration is incompatible with xbzrle");
+            return false;
+        }
+
+        if (new_caps[MIGRATION_CAPABILITY_COMPRESS]) {
+            error_setg(errp,
+                       "Mapped-ram migration is incompatible with compression");
+            return false;
+        }
+
+        if (new_caps[MIGRATION_CAPABILITY_POSTCOPY_RAM]) {
+            error_setg(errp,
+                       "Mapped-ram migration is incompatible with postcopy");
+            return false;
+        }
+    }
+
     return true;
 }