migration/postcopy: Initialize blocktime context only until listen

Before this patch, the blocktime context can be created very early, because
postcopy_ram_supported_by_host() <- migrate_caps_check() can happen during
migration object init.

The trick here is the blocktime context needs system vCPU information,
which seems to be possible to change after that point.  I didn't verify it,
but it doesn't sound right.

Now move it out and initialize the context only when postcopy listen
starts.  That is already during a migration so it should be guaranteed the
vCPU topology can never change on both sides.

While at it, assert that the ctx isn't created instead this time; the old
"if" trick isn't needed when we're sure it will only happen once now.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250613141217.474825-10-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

1 files changed, 10 insertions, 5 deletions

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index fe940f89b9..dd3615663f 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -325,13 +325,13 @@ static bool ufd_check_and_apply(int ufd, MigrationIncomingState *mis,
     }
 
 #ifdef UFFD_FEATURE_THREAD_ID
+    /*
+     * Postcopy blocktime conditionally needs THREAD_ID feature (introduced
+     * to Linux in 2017). Always try to enable it when QEMU is compiled
+     * with such environment.
+     */
     if (UFFD_FEATURE_THREAD_ID & supported_features) {
         asked_features |= UFFD_FEATURE_THREAD_ID;
-        if (migrate_postcopy_blocktime()) {
-            if (!mis->blocktime_ctx) {
-                mis->blocktime_ctx = blocktime_context_new();
-            }
-        }
     }
 #endif
 
@@ -1239,6 +1239,11 @@ int postcopy_ram_incoming_setup(MigrationIncomingState *mis)
         return -1;
     }
 
+    if (migrate_postcopy_blocktime()) {
+        assert(mis->blocktime_ctx == NULL);
+        mis->blocktime_ctx = blocktime_context_new();
+    }
+
     /* Now an eventfd we use to tell the fault-thread to quit */
     mis->userfault_event_fd = eventfd(0, EFD_CLOEXEC);
     if (mis->userfault_event_fd == -1) {