Re: [Qemu-devel] [PATCH 4/4] events doubts

[Pavel Dovgaluk]

> From: Paolo Bonzini [mailto:address@hidden On Behalf Of Paolo Bonzini
> On 07/10/2015 10:21, Pavel Dovgaluk wrote:
> > There are two kinds of events:
> > - read from the log and injected immediately (user input, network input)
> > - read from the log and wait for corresponding event in the queue (BH)
> >
> > We cannot inject BH event immediately because we do not have any information
> > about callback
> 
> Actually we do (indirectly, through aio_bh_call).  But that may not be
> the central issue, because...
> 
> > and to preserve consistency - BH cannot be processed before
> > it is scheduled by qemu core.
> 
> ... you are processing them differently anyway between record mode
> (where the BH is scheduled by the core) and replay (where the BH is
> called directly).

In record it also called through replay. It is scheduled into the replay
events queue and called at checkpoint, where queue is flushed.

> In fact, I don't understand what introduces the difference between
> record and replay that requires special handling of ptimers' bottom
> halves.  In both cases, the ptimer triggers at the desired time (based
> on checkpoints) and then the bottom half is called as soon as possible.
>  Why is a separate async event necessary?

We want to preserve order of all events that affect virtual machine behavior,
not only instructions execution. These events include processing of
interrupts, exceptions, and bottom halves.
That is why bottom halves are bind to checkpoints and recorded into the log.

> Because we only care about bottom halves from ptimers, their order
> should be the same for both record and replay.
> 
> If bottom halves async events could be removed, that would simplify a
> lot the code, and it would make it a lot easier to understand for me.

I added ptimer handling because replay didn't work when I removed BH queuing.

Pavel Dovgalyuk