From: James Cleverdon <jamesclv@us.ibm.com>
Reply-To: jamesclv@us.ibm.com
Organization: IBM LTC (xSeries Solutions)
To: Andi Kleen <ak@muc.de>
Subject: Re: 256 apic id for amd64
Date: Fri, 7 Jan 2005 17:30:09 -0800
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Cc: YhLu <YhLu@tyan.com>, Matt Domsch <Matt_Domsch@dell.com>,
       linux-kernel@vger.kernel.org, discuss@x86-64.org,
       suresh.b.siddha@intel.com
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Andi has already dealt with some of the coding style issues elsewhere in 
the thread.

My comment:  This is playing with fire.  We've gone to considerable 
trouble to make the boot_cpu_id independent of the physical APIC ID 
(which is what hard_smp_processor_id() returns).  Different BIOSes and 
different CPU revisions can cause the boot processor to shift.

Examples:  We have a box where the boot CPU has an APIC ID of 3.  
Another system starts with the BSP == 3, but the BIOS renumbers it to 
zero after first reassigning the original 0 CPU.  So, the APIC IDs end 
up 0, 1, 2, 4.  Yet another system assigns the IDs:  0, 1, 6, 7.

We can expect even stranger numbering schemes in the future, given that 
dual core hyperthreaded CPUs are in the pipeline.  Creating any 
dependency between CPU number and APIC ID is a _bad_ idea.


On Friday 07 January 2005 04:24 am, Andi Kleen wrote:
> On Thu, Jan 06, 2005 at 06:53:11PM -0800, YhLu wrote:
> > static unsigned int phys_pkg_id(int index_msb)
> > {
> >         return hard_smp_processor_id() >> index_msb;
> > }
> >
> > In arch/x86_64/kernel/genapic_cluster.c
> >
> > Should be changed to
> >
> > static unsigned int phys_pkg_id(int index_msb)
> > {
> >         /* physical apicid, so we need to substract offset */
> >         return (hard_smp_processor_id() - boot_cpu_id) >>
> > index_msb; }
>
> Why?
>
> If you want a patch merged you need to supply some more explanation
> please.
>
> Also cc Suresh & James for comment.
>
> -Andi

-- 
James Cleverdon
IBM LTC (xSeries Linux Solutions)
{jamesclv(Unix, preferred), cleverdj(Notes)} at us dot ibm dot comm
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