On 04/17/2012 04:51 AM, Li Yang-R58472 wrote:
>>> struct smp_ops_t smp_85xx_ops = {
>>> .kick_cpu = smp_85xx_kick_cpu,
>>> -#ifdef CONFIG_KEXEC
>>> +#ifdef CONFIG_HOTPLUG_CPU
>>> + .cpu_disable = generic_cpu_disable,
>>> + .cpu_die = generic_cpu_die,
>>> +#endif
>>> .give_timebase = smp_generic_give_timebase,
>>> .take_timebase = smp_generic_take_timebase,
>>> -#endif
>>> };
>>
>> We need to stop using smp_generic_give/take_timebase, not expand its use.
>> This stuff breaks under hypervisors where timebase can't be written. It
>> wasn't too bad before since we generally didn't enable CONFIG_KEXEC, but
>> we're more likely to want CONFIG_HOTPLUG_CPU.
>
> I understand that the guest OS shouldn't change the real timebase.
Cannot change it, and we've seen the tbsync code loop forever when it
tries (since the changes aren't taking effect).
> But no matter what timebase syncing method we are using, the timebase need to
> be changed anyway for certain features.
That's why I said to do it the way U-Boot does it.
> I think the better way should be trapping timebase modification in the
> hypervisor.
It does trap. Currently we treat it as a no-op. The only reasonable
alternative is to give the guest an exception. It is simply not allowed
for a guest to modify the timebase -- we are not going to break the
host's timebase sync. See the virtualized implementation note in
section 9.2.1 of book III-E of Power ISA 2.06B: "In virtualized
implementations, TBU and TBL are read-only."
>> Do the timebase sync the way U-Boot does -- if you find the appropriate
>> guts node in the device tree.
>
> That involves stopping timebase for a short time on all cores including the
> cores that are still online. Won't this be a potential issue?
I don't think it's a big deal in the contexts where you'd be doing
this -- at least not worse than the current situation. Just make sure
that you don't reset the timebase to zero or otherwise make a core see
the timebase go backward.
-Scott
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
