@samitouri / QOSamiQemu / commits / 51170c9063

migration: Fix possible division by zero on calc expected downtime

Commit dd4fe8844b changed the reporting of expected downtime behavior, so that the value will be calculated on-demand. One side effect on the change is QEMU will allow the calculation to happen anytime even if there's no transfer happening for a short while. PeterM reported an ubsan report from clang when running migration-test with aarch64 binary on x86_64 hosts. I can also reproduce if I run the test concurrently so some of the src QEMU may not get chance to push any data, causing mbps to be 0: ../migration/migration.c:1051:12: runtime error: -nan is outside the range of representable values of type 'long' Fix it by properly handle both Inf and Nan to return INT64_MAX. Add a rich comment, per PeterM's suggestion. Link: https://lore.kernel.org/r/CAFEAcA-MYH6C39xO0OLx4-M5pKurJpurwRsMqZe9q=W-NShAbw@mail.gmail.com Reported-by: Peter Maydell <peter.maydell@linaro.org> Fixes: dd4fe8844b ("migration: Calculate expected downtime on demand") Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Link: https://lore.kernel.org/r/20260511182432.1333467-1-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com>

Peter Xu committed May 11, 2026 at 14:24 UTC 51170c9063175d02dbaf8ebfc4506b81cc943d32
1 file changed +19 -1
migration/migration.c
+19 -1
@@ -63,6 +63,7 @@
63 #include "system/dirtylimit.h"
64 #include "qemu/sockets.h"
65 #include "system/kvm.h"
66 +#include "math.h"
67
68 #define NOTIFIER_ELEM_INIT(array, elem) \
69 [elem] = NOTIFIER_WITH_RETURN_LIST_INITIALIZER((array)[elem])
@@ -1044,12 +1045,29 @@ static bool migrate_show_downtime(MigrationState *s)
1045 /* Return expected downtime (unit: milliseconds) */
1046 int64_t migration_downtime_calc_expected(MigrationState *s)
1047 {
1048 + double expected_ms;
1049 +
1050 if (mig_stats.dirty_sync_count <= 1) {
1051 return migrate_downtime_limit();
1052 }
1053
1051 - return mig_stats.dirty_bytes_last_sync /
1054 + expected_ms = mig_stats.dirty_bytes_last_sync /
1055 migration_get_switchover_bw(s) * 1000;
1056 +
1057 + /*
1058 + * If we haven't been able to transfer any data, the result here could
1059 + * be NaN (for 0 / 0) or infinity (something else / 0).
1060 + *
1061 + * Return INT64_MAX as our best approximation to "this will take
1062 + * forever to complete". If the problem is transient (e.g. we just
1063 + * haven't started to transfer yet) we'll recalculate to a more
1064 + * accurate figure later.
1065 + */
1066 + if (isnan(expected_ms) || expected_ms >= (double)INT64_MAX) {
1067 + return INT64_MAX;
1068 + }
1069 +
1070 + return (int64_t) expected_ms;
1071 }
1072
1073 static void populate_time_info(MigrationInfo *info, MigrationState *s)