Re: [Qemu-devel] [RFC] Introduce vm_stop_permanent()

[Luiz Capitulino <lcapitulino@redhat.com>]

On Thu, 28 Jul 2011 20:04:58 +0200
Jan Kiszka <jan.kiszka@web.de> wrote:

> On 2011-07-28 20:00, Luiz Capitulino wrote:
> > On Thu, 28 Jul 2011 19:52:31 +0200
> > Jan Kiszka <jan.kiszka@web.de> wrote:
> > 
> >> On 2011-07-28 19:48, Luiz Capitulino wrote:
> >>> On Thu, 28 Jul 2011 14:39:23 -0300
> >>> Luiz Capitulino <lcapitulino@redhat.com> wrote:
> >>>
> >>>> On Thu, 28 Jul 2011 17:20:41 +0200
> >>>> Jan Kiszka <jan.kiszka@web.de> wrote:
> >>>>
> >>>>> On 2011-07-28 17:18, Luiz Capitulino wrote:
> >>>>>> On Thu, 28 Jul 2011 16:19:19 +0200
> >>>>>> Jan Kiszka <jan.kiszka@web.de> wrote:
> >>>>>>
> >>>>>>> On 2011-07-28 15:37, Avi Kivity wrote:
> >>>>>>>> On 07/28/2011 04:31 PM, Luiz Capitulino wrote:
> >>>>>>>>> On Thu, 28 Jul 2011 10:23:22 +0300
> >>>>>>>>> Avi Kivity<avi@redhat.com>  wrote:
> >>>>>>>>>
> >>>>>>>>>>  On 07/28/2011 12:44 AM, Blue Swirl wrote:
> >>>>>>>>>>  >  On Wed, Jul 27, 2011 at 9:42 PM, Luiz
> >>>>>>>>> Capitulino<lcapitulino@redhat.com>   wrote:
> >>>>>>>>>>  >  >   This function should be used when the VM is not supposed to
> >>>>>>>>> resume
> >>>>>>>>>>  >  >   execution (eg. by issuing 'cont' monitor command).
> >>>>>>>>>>  >  >
> >>>>>>>>>>  >  >   Today, we allow the user to resume execution even when:
> >>>>>>>>>>  >  >
> >>>>>>>>>>  >  >     o the guest shuts down and -no-shutdown is used
> >>>>>>>>>>  >  >     o there's a kvm internal error
> >>>>>>>>>>  >  >     o loading the VM state with -loadvm or "loadvm" in the
> >>>>>>>>> monitor fails
> >>>>>>>>>>  >  >
> >>>>>>>>>>  >  >   I think only badness can happen from the cases above.
> >>>>>>>>>>  >
> >>>>>>>>>>  >  I'd suppose a system_reset should bring the system back to
> >>>>>>>>> sanity and
> >>>>>>>>>>  >  then clear vm_permanent_stopped (where's -ly?)
> >>>>>>>>>
> >>>>>>>>> What's -ly?
> >>>>>>>>>
> >>>>>>>>
> >>>>>>>> permanent-ly.
> >>>>>>>>
> >>>>>>>>>>  >  except maybe for KVM
> >>>>>>>>>>  >  internal error if that can't be recovered. Then it would not very
> >>>>>>>>>>  >  permanent anymore, so the name would need adjusting.
> >>>>>>>>>>
> >>>>>>>>>>  Currently, all kvm internal errors are recoverable by reset (and
> >>>>>>>>>>  possibly by fiddling with memory/registers).
> >>>>>>>>>
> >>>>>>>>> Ok, but a poweroff in the guest isn't recoverable with system_reset
> >>>>>>>>> right? Or does it depend on the guest?
> >>>>>>>>
> >>>>>>>> Right, it's not recoverable if you shut the power down where the tractor
> >>>>>>>> beam is coupled to the main reactor.
> >>>>>>>
> >>>>>>> system_reset will bring all emulated devices back into their power-on
> >>>>>>> state - unless we have remaining bugs to fix. Actually, one may consider
> >>>>>>> issuing this reset automatically on vm_start after "permant" vm_stop.
> >>>>
> >>>> The only permanent vm_stop we'd have is poweroff when -no-shutdown is used.
> >>>>
> >>>> Are you saying that system_reset should be able to recover from that too?
> >>>
> >>> It already does, so we don't have permanent stops.
> >>
> >> Exactly. We just have stops over inconsistent states that require a
> >> reset to continue with anything useful.
> > 
> > Yes. If I got you right, you suggest that we do the reset automatically.
> > 
> > I think it's better to let the user do it, because s/he might want to
> > do something else before resetting. For example, for the kvm error the
> > user might want to save the vm state.
> 
> Associating the reset with a cont means requesting an explicit action
> from the user. I'm not suggesting to do the reset when the stop state is
> entered.

I see. But automatically resetting on cont might be unexpected to the
user, even on a bad state.

Another option would be to add a force option to cont, where the reset is
done when the state is invalid (otherwise cont will return an error).

I still prefer to let the user do it manually though.

> > For the poweroff case with -no-shutdown it's probably fine, but I don't
> > want to hard code special cases. It's better and easier to treat them all
> > as "require system_reset to recover".
> 
> In any case, we need to tag the current state as stopped-and-invalid or
> so vs. a normal stop. That remains a valuable first step. How to deal
> with that information is the second one.