Re: [Qemu-devel] [PATCH 2/7] Replace VMSTOP macros with a proper QemuState type

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 08 Aug 2011 14:22:14 +0300
Avi Kivity <avi@redhat.com> wrote:

> On 08/04/2011 01:17 PM, Jan Kiszka wrote:
> > >
> > >  Why "QemuState"?  In general, "qemu" can be inferred from the fact that
> > >  we're in qemu.git.  Suggest "RunState".
> > >
> > >  Second, these states can coexist.  A user may pause the VM
> > >  simultaneously with the watchdog firing or entering premigrate state.
> > >  In fact, with multiple monitors, each monitor can pause and resume the
> > >  vm independently.
> > >
> > >  So I think we should keep a reference count instead of just a start/stop
> > >  state.  Perhaps
> > >
> > >  vm_stop(QemuState s)
> > >  {
> > >      ++stopcount[s];
> > >  }
> > >
> > >  vm_is_stopped()
> > >  {
> > >      for (s in states)
> > >            if (stopcount[s])
> > >                return true;
> > >      return false;
> > >  }
> >
> > I don't think this makes sense nor is user-friendly. If one command
> > channel suspends the machine, others have the chance to subscribe for
> > that event.
> 
> It's inherently racy.
> 
> > Maintaining a suspension counter would mean you also need a
> > channel to query its value.
> 
> Why?
> 
> > IMHO, there is also no use for defining stopped orthogonally to
> > premigrate and other states that imply that the machine is stopped.
> > Basically they mean "stopped for/because of X". We just need to avoid
> > that you can enter plain stopped state from them by issuing the
> > corresponding monitor command. The other way around might be possible,
> > though, if there are race windows.
> >
> 
> I'm worried about the following race:
> 
>    stop
>    (qemu stopped for internal reason)
>    stop comment processed
> 
>    resume
> 
> The (qemu stopped for internal reason) part is lost.

If the "stop" you're referring to happens through vm_stop(), then no,
it won't be lost because do_vm_stop() doesn't allow qemu to be stopped
twice.