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

[Luiz Capitulino <lcapitulino@redhat.com>]

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.

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".