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Re: [Qemu-ppc] [PATCH v3] spapr: disable decrementer during reset
From: |
David Gibson |
Subject: |
Re: [Qemu-ppc] [PATCH v3] spapr: disable decrementer during reset |
Date: |
Tue, 18 Jul 2017 16:50:55 +1000 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
On Tue, Jul 18, 2017 at 10:53:01AM +0530, Nikunj A Dadhania wrote:
> David Gibson <address@hidden> 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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