Re: [Qemu-devel] [PATCH v3 0/4] Introduce attributes for timers subsystem and remove QEMU_CLOCK_VIRTUAL_EXT clock type

[Artem Pisarenko <artem.k.pisarenko@gmail.com>]

ср, 16 янв. 2019 г. в 12:15, Pavel Dovgalyuk <dovgaluk@ispras.ru>:
>
> > From: Artem Pisarenko [mailto:artem.k.pisarenko@gmail.com]
> > > It seems, that this approach is not always correct.
> > > Now timerlist_deadline_ns uses all virtual timers for deadline calculation (including
> > external
> > > ones).
> > > qemu_start_warp_timer uses the deadline for setting warp timer (which should be
> > deterministic).
> > > Therefore warp timer may become non-deterministic and replay may behave differently compared
> > to
> > > recording phase.
> > >
> > > We have to rollback these or improve somehow to avoid non-determinism.
> >
> > I dont understand how this approach would even introduce
> > non-determinism. I'm not sure about aspects of timers subsystem you
> > mentioned, assuming that maybe we missed something when tried to
> > optimize. But this has nothing to do with determinism as long as we
> > treat all virtual clock timers as deterministic, regardless of
> > EXTERNAL attribute being set or not. They are intended to be used as
> > such by design, aren't? This attribute was introduced purely to avoid
> > extra events in log.
>
> Right, but deadline calculation and icount warp events (that are written into the log too)
> depend on the active virtual timers. Therefore external ones may affect icount warp
> operation sequence.
>
> Pavel Dovgalyuk
>

Sorry, but I still don't understand the source of non-determinism.
>From what you said I may understand that replay module actually has
some additional non-trivial (for me) dependencies on EXTERNAL
attribute other than one, introduced in 'timerlist_run_timers()' (i.e.
it expects deadline calculations somewhere else to match decisions
made in this function, or something like that). I would agree that
these patch series might introduce some incorrect behavior. But it
must be identical in all emulations running in same conditions,
because deadline, warp timer and their effects are all dependent only
on virtual timers, and, therefore, are deterministic too. Of course,
it needs to be fixed. Did you find solution ?