migration: Change zero_copy_send from migration parameter to migration capability

When originally implemented, zero_copy_send was designed as a Migration
paramenter.

But taking into account how is that supposed to work, and how
the difference between a capability and a parameter, it only makes sense
that zero-copy-send would work better as a capability.

Taking into account how recently the change got merged, it was decided
that it's still time to make it right, and convert zero_copy_send into
a Migration capability.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: always define the capability, even on non-Linux but error if
set; avoids build problems with the capability

migration/migration.c

@@ -163,7 +163,8 @@ INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
     MIGRATION_CAPABILITY_COMPRESS,
     MIGRATION_CAPABILITY_XBZRLE,
     MIGRATION_CAPABILITY_X_COLO,
-    MIGRATION_CAPABILITY_VALIDATE_UUID);
+    MIGRATION_CAPABILITY_VALIDATE_UUID,
+    MIGRATION_CAPABILITY_ZERO_COPY_SEND);
 
 /* When we add fault tolerance, we could have several
    migrations at once.  For now we don't need to add
@@ -910,10 +911,6 @@ MigrationParameters *qmp_query_migrate_parameters(Error **errp)
     params->multifd_zlib_level = s->parameters.multifd_zlib_level;
     params->has_multifd_zstd_level = true;
     params->multifd_zstd_level = s->parameters.multifd_zstd_level;
-#ifdef CONFIG_LINUX
-    params->has_zero_copy_send = true;
-    params->zero_copy_send = s->parameters.zero_copy_send;
-#endif
     params->has_xbzrle_cache_size = true;
     params->xbzrle_cache_size = s->parameters.xbzrle_cache_size;
     params->has_max_postcopy_bandwidth = true;
@@ -1275,6 +1272,24 @@ static bool migrate_caps_check(bool *cap_list,
         }
     }
 
+#ifdef CONFIG_LINUX
+    if (cap_list[MIGRATION_CAPABILITY_ZERO_COPY_SEND] &&
+        (!cap_list[MIGRATION_CAPABILITY_MULTIFD] ||
+         migrate_use_compression() ||
+         migrate_use_tls())) {
+        error_setg(errp,
+                   "Zero copy only available for non-compressed non-TLS multifd migration");
+        return false;
+    }
+#else
+    if (cap_list[MIGRATION_CAPABILITY_ZERO_COPY_SEND]) {
+        error_setg(errp,
+                   "Zero copy currently only available on Linux");
+        return false;
+    }
+#endif
+
+
     /* incoming side only */
     if (runstate_check(RUN_STATE_INMIGRATE) &&
         !migrate_multi_channels_is_allowed() &&
@@ -1497,16 +1512,6 @@ static bool migrate_params_check(MigrationParameters *params, Error **errp)
         error_prepend(errp, "Invalid mapping given for block-bitmap-mapping: ");
         return false;
     }
-#ifdef CONFIG_LINUX
-    if (params->zero_copy_send &&
-        (!migrate_use_multifd() ||
-         params->multifd_compression != MULTIFD_COMPRESSION_NONE ||
-         (params->tls_creds && *params->tls_creds))) {
-        error_setg(errp,
-                   "Zero copy only available for non-compressed non-TLS multifd migration");
-        return false;
-    }
-#endif
     return true;
 }
 
@@ -1580,11 +1585,6 @@ static void migrate_params_test_apply(MigrateSetParameters *params,
     if (params->has_multifd_compression) {
         dest->multifd_compression = params->multifd_compression;
     }
-#ifdef CONFIG_LINUX
-    if (params->has_zero_copy_send) {
-        dest->zero_copy_send = params->zero_copy_send;
-    }
-#endif
     if (params->has_xbzrle_cache_size) {
         dest->xbzrle_cache_size = params->xbzrle_cache_size;
     }
@@ -1697,11 +1697,6 @@ static void migrate_params_apply(MigrateSetParameters *params, Error **errp)
     if (params->has_multifd_compression) {
         s->parameters.multifd_compression = params->multifd_compression;
     }
-#ifdef CONFIG_LINUX
-    if (params->has_zero_copy_send) {
-        s->parameters.zero_copy_send = params->zero_copy_send;
-    }
-#endif
     if (params->has_xbzrle_cache_size) {
         s->parameters.xbzrle_cache_size = params->xbzrle_cache_size;
         xbzrle_cache_resize(params->xbzrle_cache_size, errp);
@@ -2593,7 +2588,7 @@ bool migrate_use_zero_copy_send(void)
 
     s = migrate_get_current();
 
-    return s->parameters.zero_copy_send;
+    return s->enabled_capabilities[MIGRATION_CAPABILITY_ZERO_COPY_SEND];
 }
 #endif
 
@@ -4249,10 +4244,6 @@ static Property migration_properties[] = {
     DEFINE_PROP_UINT8("multifd-zstd-level", MigrationState,
                       parameters.multifd_zstd_level,
                       DEFAULT_MIGRATE_MULTIFD_ZSTD_LEVEL),
-#ifdef CONFIG_LINUX
-    DEFINE_PROP_BOOL("zero_copy_send", MigrationState,
-                      parameters.zero_copy_send, false),
-#endif
     DEFINE_PROP_SIZE("xbzrle-cache-size", MigrationState,
                       parameters.xbzrle_cache_size,
                       DEFAULT_MIGRATE_XBZRLE_CACHE_SIZE),
@@ -4290,6 +4281,10 @@ static Property migration_properties[] = {
     DEFINE_PROP_MIG_CAP("x-multifd", MIGRATION_CAPABILITY_MULTIFD),
     DEFINE_PROP_MIG_CAP("x-background-snapshot",
             MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT),
+#ifdef CONFIG_LINUX
+    DEFINE_PROP_MIG_CAP("x-zero-copy-send",
+            MIGRATION_CAPABILITY_ZERO_COPY_SEND),
+#endif
 
     DEFINE_PROP_END_OF_LIST(),
 };
@@ -4350,9 +4345,6 @@ static void migration_instance_init(Object *obj)
     params->has_multifd_compression = true;
     params->has_multifd_zlib_level = true;
     params->has_multifd_zstd_level = true;
-#ifdef CONFIG_LINUX
-    params->has_zero_copy_send = true;
-#endif
     params->has_xbzrle_cache_size = true;
     params->has_max_postcopy_bandwidth = true;
     params->has_max_cpu_throttle = true;

monitor/hmp-cmds.c

@@ -1311,12 +1311,6 @@ void hmp_migrate_set_parameter(Monitor *mon, const QDict *qdict)
         p->has_multifd_zstd_level = true;
         visit_type_uint8(v, param, &p->multifd_zstd_level, &err);
         break;
-#ifdef CONFIG_LINUX
-    case MIGRATION_PARAMETER_ZERO_COPY_SEND:
-        p->has_zero_copy_send = true;
-        visit_type_bool(v, param, &p->zero_copy_send, &err);
-        break;
-#endif
     case MIGRATION_PARAMETER_XBZRLE_CACHE_SIZE:
         p->has_xbzrle_cache_size = true;
         if (!visit_type_size(v, param, &cache_size, &err)) {

qapi/migration.json

@@ -461,6 +461,13 @@
 #                       procedure starts. The VM RAM is saved with running VM.
 #                       (since 6.0)
 #
+# @zero-copy-send: Controls behavior on sending memory pages on migration.
+#                  When true, enables a zero-copy mechanism for sending
+#                  memory pages, if host supports it.
+#                  Requires that QEMU be permitted to use locked memory
+#                  for guest RAM pages.
+#                  (since 7.1)
+#
 # Features:
 # @unstable: Members @x-colo and @x-ignore-shared are experimental.
 #
@@ -474,7 +481,8 @@
            'block', 'return-path', 'pause-before-switchover', 'multifd',
            'dirty-bitmaps', 'postcopy-blocktime', 'late-block-activate',
            { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
-           'validate-uuid', 'background-snapshot'] }
+           'validate-uuid', 'background-snapshot',
+           'zero-copy-send'] }
 
 ##
 # @MigrationCapabilityStatus:
@@ -738,12 +746,6 @@
 #                      will consume more CPU.
 #                      Defaults to 1. (Since 5.0)
 #
-# @zero-copy-send: Controls behavior on sending memory pages on migration.
-#                  When true, enables a zero-copy mechanism for sending
-#                  memory pages, if host supports it.
-#                  Requires that QEMU be permitted to use locked memory
-#                  for guest RAM pages.
-#                  Defaults to false. (Since 7.1)
 #
 # @block-bitmap-mapping: Maps block nodes and bitmaps on them to
 #                        aliases for the purpose of dirty bitmap migration.  Such
@@ -784,7 +786,6 @@
            'xbzrle-cache-size', 'max-postcopy-bandwidth',
            'max-cpu-throttle', 'multifd-compression',
            'multifd-zlib-level' ,'multifd-zstd-level',
-           { 'name': 'zero-copy-send', 'if' : 'CONFIG_LINUX'},
            'block-bitmap-mapping' ] }
 
 ##
@@ -911,13 +912,6 @@
 #                      will consume more CPU.
 #                      Defaults to 1. (Since 5.0)
 #
-# @zero-copy-send: Controls behavior on sending memory pages on migration.
-#                  When true, enables a zero-copy mechanism for sending
-#                  memory pages, if host supports it.
-#                  Requires that QEMU be permitted to use locked memory
-#                  for guest RAM pages.
-#                  Defaults to false. (Since 7.1)
-#
 # @block-bitmap-mapping: Maps block nodes and bitmaps on them to
 #                        aliases for the purpose of dirty bitmap migration.  Such
 #                        aliases may for example be the corresponding names on the
@@ -972,7 +966,6 @@
             '*multifd-compression': 'MultiFDCompression',
             '*multifd-zlib-level': 'uint8',
             '*multifd-zstd-level': 'uint8',
-            '*zero-copy-send': { 'type': 'bool', 'if': 'CONFIG_LINUX' },
             '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ] } }
 
 ##
@@ -1119,13 +1112,6 @@
 #                      will consume more CPU.
 #                      Defaults to 1. (Since 5.0)
 #
-# @zero-copy-send: Controls behavior on sending memory pages on migration.
-#                  When true, enables a zero-copy mechanism for sending
-#                  memory pages, if host supports it.
-#                  Requires that QEMU be permitted to use locked memory
-#                  for guest RAM pages.
-#                  Defaults to false. (Since 7.1)
-#
 # @block-bitmap-mapping: Maps block nodes and bitmaps on them to
 #                        aliases for the purpose of dirty bitmap migration.  Such
 #                        aliases may for example be the corresponding names on the
@@ -1178,7 +1164,6 @@
             '*multifd-compression': 'MultiFDCompression',
             '*multifd-zlib-level': 'uint8',
             '*multifd-zstd-level': 'uint8',
-            '*zero-copy-send': { 'type': 'bool', 'if': 'CONFIG_LINUX' },
             '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ] } }
 
 ##