Re: [Qemu-devel] [PATCH v3 3/5] replay: introduce new checkpoint for icount warp

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> On 01/03/2016 12:07, Pavel Dovgalyuk wrote:
> > qemu_clock_warp function is called to update virtual clock when CPU
> > is sleeping. This function includes replay checkpoint to make execution
> > deterministic in icount mode.
> > Record/replay module flushes async event queue at checkpoints.
> > Some of the events (e.g., block devices operations) include interaction
> > with hardware. E.g., APIC polled by block devices sets one of IRQ flags.
> > Flag to be set depends on currently executed thread (CPU or iothread).
> > Therefore in replay mode we have to process the checkpoints in the same thread
> > as they were recorded.
> > qemu_clock_warp function (and its checkpoint) may be called from different
> > thread. This patch introduces new checkpoint which distinguished warp
> > checkpoint calls from different threads.
> >
> > Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
> 
> I think we need two different kinds of "warp" behavior, one to start the
> warp timer (from the main loop and when a timer is set) and one to end it
> (from the CPUs).
> 
> Then the need for two checkpoints is much clearer, though I suggestnaming
> them without a reference to TCG; something like CHECKPOINT_CLOCK_WARP_START
> and CHECKPOINT_CLOCK_WARP_ACCOUNT for example.

Thanks, this seems reasonable.

> The start would be where you call qemu_clock_warp(QEMU_CLOCK_VIRTUAL, false):
> 
>     if (!use_icount) {
>         return;
>     }
>     if (!runstate_is_running()) {
>         return;
>     }
>     if (!replay_checkpoint(CHECKPOINT_CLOCK_WARP_START)) {
>         return;
>     }
>     /* I think calling icount_warp_rt here is unnecessary.  */
>     if (!all_cpu_threads_idle()) {
>         return;
>     }
>     if (qtest_enabled()) {
>         /* When testing, qtest commands advance icount.  */
>         return;
>     }
> 
>     /* We want to use the earliest deadline from ALL vm_clocks */
>     clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_RT);
>     deadline = qemu_clock_deadline_ns_all(QEMU_CLOCK_VIRTUAL);
>     ...
> 
> The end or account function, instead, would be called from tcg_exec_all()
> and also from icount_dummy_timer() (this is what makes the call to
> icount_warp_rt unnecessary above):

Why icount_warp_rt is unnecessary? There is no code to proceed the virtual clock.
Then qemu_start_warp_timer will forever setup the timer without any progress.

Pavel Dovgalyuk