[Qemu-devel] [PATCH v5 26/28] migration: allow migrate_cancel to pause postcopy

[Peter Xu <peterx@redhat.com>]

It was allowed in the past to even cancel a postcopy migration, but it
does not really make sense, and no one should be using it, since
cancelling a migration during postcopy means crashing the VM at no time.

Let's just use re-use this command as a way to pause the postcopy
migration when we detected that we are in postcopy migration.  This can
be used when we want to trigger the postcopy recovery manually.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 hmp-commands.hx       |  8 ++++++--
 migration/migration.c | 18 +++++++++++++++++-
 qapi/migration.json   |  3 ++-
 3 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/hmp-commands.hx b/hmp-commands.hx
index ffcdc34652..32fdd52212 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -952,14 +952,18 @@ ETEXI
         .name       = "migrate_cancel",
         .args_type  = "",
         .params     = "",
-        .help       = "cancel the current VM migration",
+        .help       = "cancel the current VM precopy migration, or "
+                      "pause the migration if it's in postcopy state, "
+                      "so that a postcopy recovery can be carried out later.",
         .cmd        = hmp_migrate_cancel,
     },
 
 STEXI
 @item migrate_cancel
 @findex migrate_cancel
-Cancel the current VM migration.
+If during a precopy, this command cancels the migration.  If during
+postcopy, this command pauses the migration (so that a postcopy
+recovery can be carried out afterward).
 ETEXI
 
     {
diff --git a/migration/migration.c b/migration/migration.c
index 8762fad9be..0c1a783df2 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1535,7 +1535,23 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
 
 void qmp_migrate_cancel(Error **errp)
 {
-    migrate_fd_cancel(migrate_get_current());
+    MigrationState *ms = migrate_get_current();
+    int ret;
+
+    if (ms->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
+        /*
+         * Cancelling a postcopy migration does not really make sense.
+         * Here instead we pause the migration only, so another
+         * recovery can take place if needed.
+         */
+        ret = qemu_file_shutdown(ms->to_dst_file);
+        if (ret) {
+            error_setg(errp, "Failed to pause migration stream.");
+        }
+        return;
+    }
+
+    migrate_fd_cancel(ms);
 }
 
 void qmp_migrate_continue(MigrationStatus state, Error **errp)
diff --git a/qapi/migration.json b/qapi/migration.json
index 5dfac0681d..e15bb516cd 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -881,7 +881,8 @@
 ##
 # @migrate_cancel:
 #
-# Cancel the current executing migration process.
+# Cancel the current executing migration process for precopy.  For
+# postcopy, it'll pause the migration instead.
 #
 # Returns: nothing on success
 #
-- 
2.14.3