Re: [patch] genapic: optimize & fix APIC mode setup

["Siddha, Suresh B" <suresh.b.siddha@intel.com>]

On Mon, Nov 13, 2006 at 09:43:16AM +0100, Ingo Molnar wrote:
> 
> * Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:
> 
> > There is an issue of using clustered mode along with cpu hotplug. More 
> > details are at the below link.
> > 
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=113261865814107&w=2
> 
> ok, to make sure i understand this right: it is not safe to switch any 
> local APIC in the system into clustered APIC mode on the E850x chipset 
> /at all/, because if one of the CPUs gets an INIT/startup IPI message 
> its local APIC will default to logical flat mode and might confuse the 
> chipset?

"at all" is not quite correct. We are fine as long as all the cpus are up
in clustered APIC mode before the IO-APIC RTE's are programmed.

Once the IO-APIC subsystem is up and running and later if the cpu comes online
then we have a window between the INIT/startup IPI message and the place
where we program the DFR and LDR, with in which the IO-APIC will interpret
the logical mode in RTE as 'logical flat' and will probably result in missing
the interrupt.

thanks,
suresh
> 
> on large systems that have their APIC IDs set up to group CPUs amongst 
> different clusters and hence triggered cluster mode, the chipset does 
> not get confused by this, correct?
> 
> 	Ingo