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

["Pavel Dovgaluk" <Pavel.Dovgaluk@ispras.ru>]

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> On 07/10/2015 11:50, Pavel Dovgaluk wrote:
> >> From: Paolo Bonzini [mailto:paolo.bonzini@gmail.com] 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.
> 
> Ok, got it.  I still want to understand exactly the need for the init
> and reset checkpoints, and the placement of qemu_clock_warp calls, but
> apart from that the patches are good to go for 2.5.  Thanks for your
> persistence!

Init checkpoint is needed to separate initialization events (mostly coming from block
devices) from execution ones.
Reset checkpoint is used for synchronization of machine reset call.

Pavel Dovgalyuk