Re: [Qemu-devel] [RFC PATCH v5 22/31] timer: introduce new QEMU_CLOCK_VIRTUAL_RT clock

[Pavel Dovgaluk]

> From: Paolo Bonzini [mailto:address@hidden
> On 26/11/2014 11:40, Pavel Dovgalyuk wrote:
> > This patch introduces new QEMU_CLOCK_VIRTUAL_RT clock, which
> > should be used for icount warping. Separate timer is needed
> > for replaying the execution, because warping callbacks should
> > be deterministic. We cannot make realtime clock deterministic
> > because it is used for screen updates and other simulator-specific
> > actions. That is why we added new clock which is recorded and
> > replayed when needed.
> >
> > Signed-off-by: Pavel Dovgalyuk <address@hidden>
> > ---
> >  include/qemu/timer.h |    7 +++++++
> >  qemu-timer.c         |    2 ++
> >  replay/replay.h      |    4 +++-
> >  3 files changed, 12 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/qemu/timer.h b/include/qemu/timer.h
> > index 7b43331..df27157 100644
> > --- a/include/qemu/timer.h
> > +++ b/include/qemu/timer.h
> > @@ -37,12 +37,19 @@
> >   * is suspended, and it will reflect system time changes the host may
> >   * undergo (e.g. due to NTP). The host clock has the same precision as
> >   * the virtual clock.
> > + *
> > + * @QEMU_CLOCK_VIRTUAL_RT: realtime clock used for icount warp
> > + *
> > + * This clock runs as a realtime clock, but is used for icount warp
> > + * and thus should be traced with record/replay to make warp function
> > + * behave deterministically.
> >   */
> 
> I think it should also stop/restart across "stop" and "cont" commands,
> similar to QEMU_CLOCK_VIRTUAL.  This is as simple as changing
> get_clock() to cpu_get_clock().

Not so easy :)
cpu_get_clock() checks vm_clock_seqlock which is locked in icount_warp_rt().
And after locking it requests the value of QEMU_CLOCK_VIRTUAL_RT:

    seqlock_write_lock(&timers_state.vm_clock_seqlock);
    if (runstate_is_running()) {
        int64_t clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_RT);

Pavel Dovgalyuk