linux-kernel - RE: [PATCH v4 1/4] powerpc/85xx: add HOTPLUG

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Date:	Tue, 17 Apr 2012 09:51:11 +0000
From:	Li Yang-R58472 <r58472@...escale.com>
To:	Wood Scott-B07421 <B07421@...escale.com>,
	Zhao Chenhui-B35336 <B35336@...escale.com>
CC:	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 1/4] powerpc/85xx: add HOTPLUG_CPU support

> >  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.  But no matter what timebase syncing method we are using, the timebase need to be changed anyway for certain features.  I think the better way should be trapping timebase modification in the hypervisor.

> 
> 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?

- Leo 

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