[PATCH 4/5] migration: Disallow postcopy preempt to be used with compress

[Peter Xu <peterx@redhat.com>]

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.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index fb4066dfb4..07c74a79a2 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1341,6 +1341,17 @@ 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;
-- 
2.32.0