migration: Disallow postcopy preempt to be used with compress

The preempt mode requires the capability to assign channel for each of the
page, while the compression logic will currently assign pages to different
compress thread/local-channel so potentially they're incompatible.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

diff --git a/migration/migration.c b/migration/migration.c
index 739bb683f34feb23e4f627a19e86cbba9de9a53c..f3ed77a7d095e8f790171b3788ab3677102f710f 100644 (file)
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1337,6 +1337,17 @@ static bool migrate_caps_check(bool *cap_list,
             error_setg(errp, "Postcopy preempt requires postcopy-ram");
             return false;
         }
+
+        /*
+         * Preempt mode requires urgent pages to be sent in separate
+         * channel, OTOH compression logic will disorder all pages into
+         * different compression channels, which is not compatible with the
+         * preempt assumptions on channel assignments.
+         */
+        if (cap_list[MIGRATION_CAPABILITY_COMPRESS]) {
+            error_setg(errp, "Postcopy preempt not compatible with compress");
+            return false;
+        }
     }
 
     return true;