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

[Alon Levy]

On Thu, Jul 28, 2011 at 03:22:52PM -0300, Luiz Capitulino wrote:
> On Thu, 28 Jul 2011 20:04:58 +0200
> Jan Kiszka <address@hidden> wrote:
> 
> > On 2011-07-28 20:00, Luiz Capitulino wrote:
> > > On Thu, 28 Jul 2011 19:52:31 +0200
> > > Jan Kiszka <address@hidden> wrote:
> > > 
> > >> On 2011-07-28 19:48, Luiz Capitulino wrote:
> > >>> On Thu, 28 Jul 2011 14:39:23 -0300
> > >>> Luiz Capitulino <address@hidden> wrote:
> > >>>
> > >>>> On Thu, 28 Jul 2011 17:20:41 +0200
> > >>>> Jan Kiszka <address@hidden> wrote:
> > >>>>
> > >>>>> On 2011-07-28 17:18, Luiz Capitulino wrote:
> > >>>>>> On Thu, 28 Jul 2011 16:19:19 +0200
> > >>>>>> Jan Kiszka <address@hidden> 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<address@hidden>  wrote:
> > >>>>>>>>>
> > >>>>>>>>>>  On 07/28/2011 12:44 AM, Blue Swirl wrote:
> > >>>>>>>>>>  >  On Wed, Jul 27, 2011 at 9:42 PM, Luiz
> > >>>>>>>>> Capitulino<address@hidden>   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.

I use this feature a lot in testing, i.e. that there is no automatic reset
when I run a vm with -no-shutdown and I do a guest initiated shutdown.
Very convenient when running with snapshot - great time to do commit ide0.
I have a patch I send a month or so ago to make sure you can do this endlessly -
currently after the first shutdown the -no-shutdown set flag is reset.

> 
> 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.
>