Re: [PULL 13/23] migration: Calculate expected downtime on demand

[Peter Xu <peterx@redhat.com>]

On Thu, May 07, 2026 at 08:57:24PM +0100, Peter Maydell wrote:
> On Tue, 5 May 2026 at 21:29, Peter Xu <peterx@redhat.com> wrote:
> >
> > This value does not need to be calculated as frequent.  Only calculate it
> > on demand when query-migrate happened.  With that we can remove the
> > variable in MigrationState.
> >
> > This paves way for fixing this value to include all modules (not only RAM
> > but others too).
> >
> > Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
> > Link: https://lore.kernel.org/r/20260421202110.306051-12-peterx@redhat.com
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> Hi; I'm seeing a clang undefined-behaviour sanitizer failure
> in the code introduced in this change when running the
> aarch64 migration-test via "make check" on an x86-64 host.
> 
> It seems to happen fairly reliably when I do a "make check -j20",
> but not when I run the test on its own, so it's probably load dependent.
> 
> Here's the backtrace:
> 
> ../../migration/migration.c:1051:12: runtime error: inf is outside the
> range of representable values of type 'long'
>     #0 0x57b49d635c0d in migration_downtime_calc_expected
> /home/pm215/qemu/build/arm-clang/../../migration/migration.c:1051:12
>     #1 0x57b49d63e860 in populate_time_info
> /home/pm215/qemu/build/arm-clang/../../migration/migration.c:1075:35
>     #2 0x57b49d63617e in fill_source_migration_info
> /home/pm215/qemu/build/arm-clang/../../migration/migration.c:1184:9
>     #3 0x57b49d63617e in qmp_query_migrate
> /home/pm215/qemu/build/arm-clang/../../migration/migration.c:1264:5
>     #4 0x57b49e4aed75 in qmp_marshal_query_migrate
> /home/pm215/qemu/build/arm-clang/qapi/qapi-commands-migration.c:48:14
>     #5 0x57b49e526814 in do_qmp_dispatch_bh
> /home/pm215/qemu/build/arm-clang/../../qapi/qmp-dispatch.c:128:5
>     #6 0x57b49e58c35a in aio_bh_call
> /home/pm215/qemu/build/arm-clang/../../util/async.c:173:5
>     #7 0x57b49e58c698 in aio_bh_poll
> /home/pm215/qemu/build/arm-clang/../../util/async.c:220:13
>     #8 0x57b49e542fc1 in aio_dispatch
> /home/pm215/qemu/build/arm-clang/../../util/aio-posix.c:390:5
>     #9 0x57b49e58f10a in aio_ctx_dispatch
> /home/pm215/qemu/build/arm-clang/../../util/async.c:365:5
>     #10 0x7c74a09b8584
> (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x5d584) (BuildId:
> 116e142b9b52c8a4dfd403e759e71ab8f95d8bb3)
>     #11 0x7c74a09b86cf in g_main_context_dispatch
> (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x5d6cf) (BuildId:
> 116e142b9b52c8a4dfd403e759e71ab8f95d8bb3)
>     #12 0x57b49e5901cb in glib_pollfds_poll
> /home/pm215/qemu/build/arm-clang/../../util/main-loop.c:290:9
>     #13 0x57b49e5901cb in os_host_main_loop_wait
> /home/pm215/qemu/build/arm-clang/../../util/main-loop.c:313:5
>     #14 0x57b49e5901cb in main_loop_wait
> /home/pm215/qemu/build/arm-clang/../../util/main-loop.c:592:11
>     #15 0x57b49d5f5486 in qemu_main_loop
> /home/pm215/qemu/build/arm-clang/../../system/runstate.c:948:9
>     #16 0x57b49e42cdfb in qemu_default_main
> /home/pm215/qemu/build/arm-clang/../../system/main.c:50:14
>     #17 0x57b49e42cdd3 in main
> /home/pm215/qemu/build/arm-clang/../../system/main.c:93:9
> 
> 
> > +/* Return expected downtime (unit: milliseconds) */
> > +int64_t migration_downtime_calc_expected(MigrationState *s)
> > +{
> > +    if (mig_stats.dirty_sync_count <= 1) {
> > +        return migrate_downtime_limit();
> > +    }
> > +
> > +    return mig_stats.dirty_bytes_last_sync /
> > +        migration_get_switchover_bw(s) * 1000;
> > +}
> 
> Presumably in this function migration_get_switchover_bw() returns 0,
> so the (floating-point) division results in Infinity. That's fine
> until we have to convert it to int64_t to return it, which is the
> UB that the sanitizer is complaining about...

True, I can easily reproduce the warning too.  I'll send a patch.

Thanks,

-- 
Peter Xu