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

[Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>]

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.

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

Regards
Nikunj