Re: [Qemu-devel] [PATCH] monitor: fix crash for platforms without a CPU 0

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Sep 21, 2016 at 09:14:02AM -0300, Eduardo Habkost wrote:
> On Wed, Sep 21, 2016 at 03:29:26PM +1000, David Gibson wrote:
> > Now that we allow CPU hot unplug on a few platforms, we can end up in a
> > situation where we don't have a CPU with index 0.  Or at least we could,
> > if we didn't have code to explicitly prohibit unplug of CPU 0.
> > 
> > Longer term we want to allow CPU 0 unplug, this patch is an early step in
> > allowing this, by removing an assumption in the monitor code that CPU 0
> > always exists.
> > 
> > Signed-off-by: Cédric Le Goater <clg@kaod.org>
> > [dwg: Rewrote commit message to better explain background]
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  monitor.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > Anyone want to volunteer to take this through their tree?  If not, I
> > can take it through my ppc tree.
> > 
> > diff --git a/monitor.c b/monitor.c
> > index 8bb8bbf..83c4edf 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -1025,7 +1025,7 @@ int monitor_set_cpu(int cpu_index)
> >  CPUState *mon_get_cpu(void)
> >  {
> >      if (!cur_mon->mon_cpu) {
> > -        monitor_set_cpu(0);
> > +        monitor_set_cpu(first_cpu->cpu_index);
> 
> So, we are replacing the "CPU 0 always exists" assumption with a
> "first_cpu is always non-NULL" assumption.

Well, we're replacing "CPU 0 is always present" assumption with "At
least one CPU is always present", which is a strictly weaker
constraint.

> But considering that the first_cpu assumption already exists
> elsewhere and those cases can be found easily using grep, I think
> this is OK. So:
> 
> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
> 
> BTW, it is also possible to crash QEMU by unplugging the current
> monitor CPU;

Ah... good point.

>   (qemu) device_add qemu64-x86_64-cpu,socket-id=2,core-id=0,thread-id=0,id=mycpu
>   (qemu) cpu 2
>   (qemu) device_del mycpu
>   (qemu) info registers
>   qemu:qemu_cpu_kick_thread: No such process
>   $ 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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