[RFC 4/6] migration: Introduce dirtylimit capability

[huangy81@chinatelecom.cn]

From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>

Introduce migration dirtylimit capability, which can
be turned on before live migration and limit dirty
page rate durty live migration.

Dirtylimit dirtylimit capability is kind of like
auto-converge but using dirtylimit instead of traditional
cpu-throttle to throttle guest down.

To enable this feature, turn on the dirtylimit capability
before live migration using migratioin-set-capabilities,
and set dirtylimit-related parameters "vcpu-dirtylimit",
"vcpu-dirtylimit-period" suitably to speed up convergence.

Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
---
 migration/migration.c | 7 ++++++-
 qapi/migration.json   | 5 ++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 9e4ce01..4a659b6 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2427,7 +2427,11 @@ bool migrate_auto_converge(void)
 
 bool migrate_dirtylimit(void)
 {
-    return false;
+    MigrationState *s;
+
+    s = migrate_get_current();
+
+    return s->enabled_capabilities[MIGRATION_CAPABILITY_DIRTYLIMIT];
 }
 
 bool migrate_zero_blocks(void)
@@ -4270,6 +4274,7 @@ static Property migration_properties[] = {
     DEFINE_PROP_MIG_CAP("x-multifd", MIGRATION_CAPABILITY_MULTIFD),
     DEFINE_PROP_MIG_CAP("x-background-snapshot",
             MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT),
+    DEFINE_PROP_MIG_CAP("x-dirtylimit", MIGRATION_CAPABILITY_DIRTYLIMIT),
 
     DEFINE_PROP_END_OF_LIST(),
 };
diff --git a/qapi/migration.json b/qapi/migration.json
index 68c1fe0..30ad413 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -463,6 +463,9 @@
 #                       procedure starts. The VM RAM is saved with running VM.
 #                       (since 6.0)
 #
+# @dirtylimit: Use dirtylimit to throttle down guest if enabled.
+#              (since 7.0)
+#
 # Features:
 # @unstable: Members @x-colo and @x-ignore-shared are experimental.
 #
@@ -476,7 +479,7 @@
            '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', 'dirtylimit'] }
 
 ##
 # @MigrationCapabilityStatus:
-- 
1.8.3.1