Re: [Qemu-devel] [PATCH v3] spapr: disable decrementer during reset

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

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On Tue, Jul 18, 2017 at 10:53:01AM +0530, Nikunj A Dadhania wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
> 
> > On Mon, Jul 17, 2017 at 09:46:39AM +0530, Nikunj A Dadhania wrote:
> >> Rebooting a SMP TCG guest is broken for both single/multi threaded TCG.
> >> 
> >> When reset happens, all the CPUs are in halted state. First CPU is brought out
> >> of reset and secondary CPUs would be initialized by the guest kernel using a
> >> rtas call start-cpu.
> >> 
> >> However, in case of TCG, decrementer interrupts keep on coming and waking the
> >> secondary CPUs up.
> >> 
> >> These secondary CPUs would see the decrementer interrupt pending, which makes
> >> cpu::has_work() to bring them out of wait loop and start executing
> >> tcg_exec_cpu().
> >> 
> >> The problem with this is all the CPUs wake up and start booting SLOF image,
> >> causing the following exception(4 CPUs TCG VM):
> >
> > Ok, I'm still trying to understand why the behaviour on reboot is
> > different from the first boot.
> 
> During first boot, the cpu is in the stopped state, so
> cpus.c:cpu_thread_is_idle returns true and CPU remains in halted state
> until rtas start-cpu. Therefore, we never check the cpu_has_work()
> 
> In case of reboot, all CPUs are resumed after reboot. So we check the
> next condition cpu_has_work() in cpu_thread_is_idle(), where we see a
> DECR interrupt and remove the CPU from halted state as the CPU has
> work.

Ok, so it sounds like we should set stopped on all the secondary CPUs
on reset as well.  What's causing them to be resumed after the reset
at the moment?

> > AFAICT on initial boot, the LPCR will
> > have DEE / PECE3 enabled.  So why aren't we getting the same problem
> > then?
> 
> Regards
> Nikunj
> 

-- 
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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