Re: [patch] genapic: default to physical mode on hotplug CPU kernels

[Siddha, Suresh B]

On Tue, Nov 28, 2006 at 09:23:22PM +0100, Ingo Molnar wrote:
> 
> * Siddha, Suresh B <suresh.b.siddha@xxxxxxxxx> wrote:
> 
> > On Tue, Nov 28, 2006 at 07:33:46AM +0100, Ingo Molnar wrote:
> > > -	if (clusters <= 1 && max_cluster <= 8 && cluster_cnt[0] == max_cluster)
> > > +	if (max_apic < 8)
> > 
> > Patch mostly looks good.  Instead of checking for max_apic, can we use
> > 	cpus_weight(cpu_possible_map) <= 8
> 
> ok - but i think it's still possible the BIOS tells us APIC IDs that are 
> larger than 7, even if there are fewer CPUs. So i think the patch below 
> should cover it. Agreed?
> 

I think it is ok to use flat mode even when APIC IDs are larger than 7, as
we rely on LDR's which are programmed using smp_processor_id().

IMO, cpus_weight check should be fine.

thanks,
suresh

> 	Ingo
> 
> -------------------->
> From: Ingo Molnar <mingo@xxxxxxx>
> Subject: [patch] genapic: default to physical mode on hotplug CPU kernels
> 
> default to physical mode on hotplug CPU kernels.
> 
> Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
> 
> Index: linux/arch/x86_64/kernel/genapic.c
> ===================================================================
> --- linux.orig/arch/x86_64/kernel/genapic.c
> +++ linux/arch/x86_64/kernel/genapic.c
> @@ -51,7 +51,7 @@ void __init clustered_apic_check(void)
>  			max_apic = id;
>  	}
>  
> -	if (max_apic < 8)
> +	if (max_apic < 8 && cpus_weight(cpu_possible_map) <= 8)
>  		genapic = &apic_flat;
>  	else
>  		genapic = &apic_physflat;
-
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