Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
Ilya Leoshkevich writes:
> On Mon, 2025-12-01 at 10:36 +, Alex Bennée wrote:
>> Ilya Leoshkevich 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 +, Alex Bennée wrote:
>> > > > > Ilya Leoshkevich 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
>> > > > > 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.
>> >
>> > 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?
>
> Getting rid of it fixes the problem and makes sense anyway.
>
>> > - 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.
>
> Yes, it is armed expired.
>
> Isn't it a deficiency in record-replay that work and timers are not
> ordered relative to each other? Can't it bite us somewhere else?
They normally should be although I notice:
void icount_handle_deadline(void)
{
assert(qemu_in_vcpu_thread());
int64_t deadline = qemu_clock_deadline_ns_all(QEMU_CLOCK_VIRTUAL,
QEMU_TIMER_ATTR_ALL);
/*
* Instructions, interrupts, and exceptions are processed in cpu-exec.
* Don't interrupt cpu thread, when these events are waiting
* (i.e., there is no checkpoint)
*/
if (deadline == 0) {
icount_notify_aio_contexts();
}
}
should run the pre-expired timers before we exec the current TB. But the
comment suggests it is not expecting any checkpoint related activity. I
wonder if we can assert that is the case to catch future issues.
>> > - 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
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
On Mon, 2025-12-01 at 12:17 +0100, Ilya Leoshkevich wrote: > On Mon, 2025-12-01 at 10:36 +, Alex Bennée wrote: > > Ilya Leoshkevich 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 +, Alex Bennée wrote: > > > > > > Ilya Leoshkevich 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 > > > > > > > > > > > > > > > > > > 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 > > > > > > > > > --- > > > > > > > > > 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 [...] > > > 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? > > Getting rid of it fixes the problem and makes sense anyway. Hmm, on the other hand, this appears to have been done this way deliberately: commit 7c12f710bad60dc7e509da4e80c77e952ef0490c Author: David Hildenbrand Date: Wed Jun 27 15:44:09 2018 +0200 s390x/tcg: rearm the CKC timer during migration If the CPU data is migrated after the TOD clock, the CKC timer of a CPU is not rearmed. Let's rearm it when loading the CPU state. Introduce tcg-stub.c just like kvm-stub.c for tcg specific stubs. I guess introducing a dependency on migration order is indeed not great for maintainability. > > > - 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. > > Yes, it is armed expired. > > > Isn't it a deficiency in record-replay that work and timers are not > ordered relative to each other? Can't it bite us somewhere else? [...] >
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
On Mon, 2025-12-01 at 10:36 +, Alex Bennée wrote: > Ilya Leoshkevich 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 +, Alex Bennée wrote: > > > > > Ilya Leoshkevich 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 > > > > > > > > > > > > > > > > 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 > > > > > > > > --- > > > > > > > > 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 > > > > > > > > > > > > > > > > > > > > > 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. > > > > > >
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
Ilya Leoshkevich 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 +, Alex Bennée wrote: >> > > Ilya Leoshkevich 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 >> > > > > > >> > > > > > 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 >> > > > > > --- >> > > > > > 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 >> > > > > >> > > > > >> > > > > 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 62c
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
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 +, Alex Bennée wrote: > > > Ilya Leoshkevich 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 > > > > > > > > > > > > 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 > > > > > > --- > > > > > > 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 > > > > > > > > > > > > > > > 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, cons
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
On Sun, 2025-11-30 at 19:32 +0100, Ilya Leoshkevich wrote: > On Sun, 2025-11-30 at 16:47 +, Alex Bennée wrote: > > Ilya Leoshkevich 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 > > > > > > > > > > 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 > > > > > --- > > > > > 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 > > > > > > > > > > > > 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_cu
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
On Sun, 2025-11-30 at 16:47 +, Alex Bennée wrote: > Ilya Leoshkevich 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 > > > > > > > > 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 > > > > --- > > > > 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 > > > > > > > > > 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.
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
Ilya Leoshkevich 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 >> > >> > 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 >> > --- >> > 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 >> >> >> 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? -- Alex Bennée Virtualisation Tech Lead @ Linaro
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
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 > > > > 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 > > --- > > 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 > > > 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.
Re: [RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
On Fri, 2025-11-28 at 14:39 +0100, Thomas Huth wrote: > From: Thomas Huth > > 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 > --- > 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 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; }
[RFC PATCH] tests/functional/s390x: Add reverse debugging test for s390x
From: Thomas Huth 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 --- 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 diff --git a/tests/functional/reverse_debugging.py b/tests/functional/reverse_debugging.py index 86fca8d81f1..8b9507674a0 100644 --- a/tests/functional/reverse_debugging.py +++ b/tests/functional/reverse_debugging.py @@ -64,7 +64,7 @@ def vm_get_icount(vm): @skipIfMissingImports("pygdbmi") # Required by GDB class @skipIfMissingEnv("QEMU_TEST_GDB") -def reverse_debugging(self, gdb_arch, shift=7, args=None): +def reverse_debugging(self, gdb_arch, shift=7, args=None, big_endian=False): from qemu_test import GDB # create qcow2 for snapshots @@ -99,6 +99,8 @@ def reverse_debugging(self, gdb_arch, shift=7, args=None): gdb_cmd = os.getenv('QEMU_TEST_GDB') gdb = GDB(gdb_cmd) try: +if big_endian: +gdb.cli("set endian big") self.reverse_debugging_run(gdb, vm, port, gdb_arch, last_icount) finally: self.log.info('exiting gdb and qemu') diff --git a/tests/functional/s390x/meson.build b/tests/functional/s390x/meson.build index 70cd36e2913..0f03e1c9db8 100644 --- a/tests/functional/s390x/meson.build +++ b/tests/functional/s390x/meson.build @@ -6,6 +6,7 @@ test_s390x_timeouts = { tests_s390x_system_quick = [ 'vmstate', + 'reverse_debug', ] tests_s390x_system_thorough = [ diff --git a/tests/functional/s390x/test_reverse_debug.py b/tests/functional/s390x/test_reverse_debug.py new file mode 100755 index 000..0767dc91678 --- /dev/null +++ b/tests/functional/s390x/test_reverse_debug.py @@ -0,0 +1,21 @@ +#!/usr/bin/env python3 +# +# SPDX-License-Identifier: GPL-2.0-or-later +# +''' +Reverse debugging test for s390x +''' + +from reverse_debugging import ReverseDebugging + + +class ReverseDebuggingS390x(ReverseDebugging): + +def test_revdbg(self): +self.set_machine('s390-ccw-virtio') +self.reverse_debugging(gdb_arch='s390:64-bit', shift=6, + big_endian=True, args=('-no-shutdown',)) + + +if __name__ == '__main__': +ReverseDebugging.main() -- 2.51.1
