Ilya Leoshkevich <[email protected]> writes: > On Sun, 2025-11-30 at 20:03 +0100, Ilya Leoshkevich wrote: >> On Sun, 2025-11-30 at 19:32 +0100, Ilya Leoshkevich wrote: >> > On Sun, 2025-11-30 at 16:47 +0000, Alex Bennée wrote: >> > > Ilya Leoshkevich <[email protected]> writes: >> > > >> > > > On Fri, 2025-11-28 at 18:25 +0100, Ilya Leoshkevich wrote: >> > > > > On Fri, 2025-11-28 at 14:39 +0100, Thomas Huth wrote: >> > > > > > From: Thomas Huth <[email protected]> >> > > > > > >> > > > > > We just have to make sure that we can set the endianness to >> > > > > > big >> > > > > > endian, >> > > > > > then we can also run this test on s390x. >> > > > > > >> > > > > > Signed-off-by: Thomas Huth <[email protected]> >> > > > > > --- >> > > > > > Marked as RFC since it depends on the fix for this bug (so >> > > > > > it >> > > > > > cannot >> > > > > > be merged yet): >> > > > > > >> > > > > > https://lore.kernel.org/qemu-devel/[email protected] >> > > > > > / >> > > > > > >> > > > > > tests/functional/reverse_debugging.py | 4 +++- >> > > > > > tests/functional/s390x/meson.build | 1 + >> > > > > > tests/functional/s390x/test_reverse_debug.py | 21 >> > > > > > ++++++++++++++++++++ >> > > > > > 3 files changed, 25 insertions(+), 1 deletion(-) >> > > > > > create mode 100755 >> > > > > > tests/functional/s390x/test_reverse_debug.py >> > > > > >> > > > > Reviewed-by: Ilya Leoshkevich <[email protected]> >> > > > > >> > > > > >> > > > > I have a simple fix which helps with your original report, >> > > > > but >> > > > > not >> > > > > with this test. I'm still investigating. >> > > > > >> > > > > --- a/target/s390x/machine.c >> > > > > +++ b/target/s390x/machine.c >> > > > > @@ -52,6 +52,14 @@ static int cpu_pre_save(void *opaque) >> > > > > kvm_s390_vcpu_interrupt_pre_save(cpu); >> > > > > } >> > > > > >> > > > > + if (tcg_enabled()) { >> > > > > + /* >> > > > > + * Ensure symmetry with cpu_post_load() with respect >> > > > > to >> > > > > + * CHECKPOINT_CLOCK_VIRTUAL. >> > > > > + */ >> > > > > + tcg_s390_tod_updated(CPU(cpu), RUN_ON_CPU_NULL); >> > > > > + } >> > > > > + >> > > > > return 0; >> > > > > } >> > > > >> > > > Interestingly enough, this patch fails only under load, e.g., >> > > > if >> > > > I >> > > > run >> > > > make check -j"$(nproc)" or if I run your test in isolation, but >> > > > with >> > > > stress-ng cpu in background. The culprit appears to be: >> > > > >> > > > s390_tod_load() >> > > > qemu_s390_tod_set() >> > > > async_run_on_cpu(tcg_s390_tod_updated) >> > > > >> > > > Depending on the system load, this additional >> > > > tcg_s390_tod_updated() >> > > > may or may not end up being called during handle_backward(). If >> > > > it >> > > > does, we get an infinite loop again, because now we need two >> > > > checkpoints. >> > > > >> > > > I have a feeling that this code may be violating some record- >> > > > replay >> > > > requirement, but I can't quite put my finger on it. For >> > > > example, >> > > > async_run_on_cpu() does not sound like something deterministic, >> > > > but >> > > > then again it just queues work for rr_cpu_thread_fn(), which is >> > > > supposed to be deterministic. >> > > >> > > The the async_run_on_cpu is called from the vcpu thread in >> > > response >> > > to a >> > > deterministic event at a known point in time it should be fine. >> > > If >> > > it >> > > came from another thread that is not synchronised via replay_lock >> > > then >> > > things will go wrong. >> > > >> > > But this is a VM load save helper? >> > >> > Yes, and it's called from the main thread. Either during >> > initialization, or as a reaction to GDB packets. >> > >> > Here is the call stack: >> > >> > qemu_loadvm_state() >> > qemu_loadvm_state_main() >> > qemu_loadvm_section_start_full() >> > vmstate_load() >> > vmstate_load_state() >> > cpu_post_load() >> > tcg_s390_tod_updated() >> > update_ckc_timer() >> > timer_mod() >> > s390_tod_load() >> > qemu_s390_tod_set() # via tdc->set() >> > async_run_on_cpu(tcg_s390_tod_updated) >> > >> > So you think we may have to take the replay lock around >> > load_snapshot()? So that all async_run_on_cpu() calls it makes end >> > up >> > being handled by the vCPU thread deterministically. >> >> To answer my own question: apparently this is already the case; at >> least, the following does not cause any fallout: >> >> diff --git a/include/system/replay.h b/include/system/replay.h >> index 6859df09580..e1cd9b2f900 100644 >> --- a/include/system/replay.h >> +++ b/include/system/replay.h >> @@ -60,6 +60,7 @@ extern char *replay_snapshot; >> >> void replay_mutex_lock(void); >> void replay_mutex_unlock(void); >> +bool replay_mutex_locked(void); >> >> static inline void replay_unlock_guard(void *unused) >> { >> diff --git a/migration/savevm.c b/migration/savevm.c >> index 62cc2ce25cb..ba945d3a1ea 100644 >> --- a/migration/savevm.c >> +++ b/migration/savevm.c >> @@ -3199,6 +3199,8 @@ bool save_snapshot(const char *name, bool >> overwrite, const char *vmstate, >> uint64_t vm_state_size; >> g_autoptr(GDateTime) now = g_date_time_new_now_local(); >> >> + g_assert(replay_mutex_locked()); >> + >> GLOBAL_STATE_CODE(); >> >> if (!migrate_can_snapshot(errp)) { >> @@ -3390,6 +3392,8 @@ bool load_snapshot(const char *name, const char >> *vmstate, >> int ret; >> MigrationIncomingState *mis = migration_incoming_get_current(); >> >> + g_assert(replay_mutex_locked()); >> + >> if (!migrate_can_snapshot(errp)) { >> return false; >> } >> diff --git a/replay/replay-internal.h b/replay/replay-internal.h >> index 75249b76936..30825a0753e 100644 >> --- a/replay/replay-internal.h >> +++ b/replay/replay-internal.h >> @@ -124,7 +124,6 @@ void replay_get_array_alloc(uint8_t **buf, size_t >> *size); >> * synchronisation between vCPU and main-loop threads. */ >> >> void replay_mutex_init(void); >> -bool replay_mutex_locked(void); >> >> /*! Checks error status of the file. */ >> void replay_check_error(void); > > I believe now I at least understand what the race is about: > > - cpu_post_load() fires the TOD timer immediately. > > - s390_tod_load() schedules work for firing the TOD timer.
Is this a duplicate of work then? Could we just rely on one or the other? If you drop the cpu_post_load() tweak then the vmstate load helper should still ensure everything works right? > - If rr loop sees work and then timer, we get one timer callback. > > - If rr loop sees timer and then work, we get two timer callbacks. If the timer is armed we should expect at least to execute a few instructions before triggering the timer, unless it was armed ready expired. > - Record and replay may diverge due to this race. > > - In this particular case divergence makes rr loop spin: it sees that > TOD timer has expired, but cannot invoke its callback, because there > is no recorded CHECKPOINT_CLOCK_VIRTUAL. > > - The order in which rr loop sees work and timer depends on whether > and when rr loop wakes up during load_snapshot(). > > - rr loop may wake up after the main thread kicks the CPU and drops > the BQL, which may happen if it calls, e.g., qemu_cond_wait_bql(). -- Alex Bennée Virtualisation Tech Lead @ Linaro
