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migration: Fix possible division by zero on calc expected downtime

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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>
This commit is contained in:
Peter Xu
2026-05-11 14:24:32 -04:00
parent 1c592bf19b
commit 51170c9063
1 changed files with 19 additions and 1 deletions
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20
migration/migration.c
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@@ -63,6 +63,7 @@
#include "system/dirtylimit.h"
#include "qemu/sockets.h"
#include "system/kvm.h"
#include "math.h"
#define NOTIFIER_ELEM_INIT(array, elem) \
[elem] = NOTIFIER_WITH_RETURN_LIST_INITIALIZER((array)[elem])
@@ -1044,12 +1045,29 @@ static bool migrate_show_downtime(MigrationState *s)
/* Return expected downtime (unit: milliseconds) */
int64_t migration_downtime_calc_expected(MigrationState *s)
{
double expected_ms;
if (mig_stats.dirty_sync_count <= 1) {
return migrate_downtime_limit();
}
return mig_stats.dirty_bytes_last_sync /
expected_ms = mig_stats.dirty_bytes_last_sync /
migration_get_switchover_bw(s) * 1000;
/*
* If we haven't been able to transfer any data, the result here could
* be NaN (for 0 / 0) or infinity (something else / 0).
*
* Return INT64_MAX as our best approximation to "this will take
* forever to complete". If the problem is transient (e.g. we just
* haven't started to transfer yet) we'll recalculate to a more
* accurate figure later.
*/
if (isnan(expected_ms) || expected_ms >= (double)INT64_MAX) {
return INT64_MAX;
}
return (int64_t) expected_ms;
}
static void populate_time_info(MigrationInfo *info, MigrationState *s)