[PATCH v2 1/2] migration: enfocre multifd and postcopy preempt to be set before incoming

[Wei Wang <wei.w.wang@intel.com>]

qemu_start_incoming_migration needs to check the number of multifd
channels or postcopy ram channels to configure the backlog parameter (i.e.
the maximum length to which the queue of pending connections for sockfd
may grow) of listen(). So enforce the usage of postcopy-preempt and
multifd as below:
- need to use "-incoming defer" on the destination; and
- set_capability and set_parameter need to be done before migrate_incoming

Otherwise, disable the use of the features and report error messages to
remind users to adjust the commands.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
---
 migration/options.c | 36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

diff --git a/migration/options.c b/migration/options.c
index a560483871..954a39aa6d 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -415,6 +415,11 @@ INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
     MIGRATION_CAPABILITY_VALIDATE_UUID,
     MIGRATION_CAPABILITY_ZERO_COPY_SEND);
 
+static bool migrate_incoming_started(void)
+{
+    return !!migration_incoming_get_current()->transport_data;
+}
+
 /**
  * @migration_caps_check - check capability compatibility
  *
@@ -538,6 +543,12 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp)
             error_setg(errp, "Postcopy preempt not compatible with compress");
             return false;
         }
+
+        if (migrate_incoming_started()) {
+            error_setg(errp,
+                       "Postcopy preempt must be set before incoming starts");
+            return false;
+        }
     }
 
     if (new_caps[MIGRATION_CAPABILITY_MULTIFD]) {
@@ -545,6 +556,10 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp)
             error_setg(errp, "Multifd is not compatible with compress");
             return false;
         }
+        if (migrate_incoming_started()) {
+            error_setg(errp, "Multifd must be set before incoming starts");
+            return false;
+        }
     }
 
     return true;
@@ -998,11 +1013,22 @@ bool migrate_params_check(MigrationParameters *params, Error **errp)
 
     /* x_checkpoint_delay is now always positive */
 
-    if (params->has_multifd_channels && (params->multifd_channels < 1)) {
-        error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
-                   "multifd_channels",
-                   "a value between 1 and 255");
-        return false;
+    if (params->has_multifd_channels) {
+        if (params->multifd_channels < 1) {
+            error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
+                       "multifd_channels",
+                       "a value between 1 and 255");
+            return false;
+        }
+        if (migrate_incoming_started()) {
+            MigrationState *ms = migrate_get_current();
+
+            ms->capabilities[MIGRATION_CAPABILITY_MULTIFD] = false;
+            error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
+                       "multifd_channels",
+                       "must be set before incoming starts");
+            return false;
+        }
     }
 
     if (params->has_multifd_zlib_level &&
-- 
2.27.0