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: C├ędric Le Goater <c...@kaod.org>
> [dwg: Rewrote commit message to better explain background]
> Signed-off-by: David Gibson <da...@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.

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 <ehabk...@redhat.com>

BTW, it is also possible to crash QEMU by unplugging the current
monitor CPU;

  (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
  $ 

-- 
Eduardo

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