From: James Cleverdon On summit-based machines the cpu_sibling_map data has been hosed for some time. I found out why in Intel's IA-32 Software Deveveopers' Manual Vol 2 under CPUID. Looks like the value that cpuid returns is the one latched at reset, and doesn't reflect any changes made by the BIOS later: * Local APIC ID (high byte of EBX)--this number is the 8-bit ID that is assigned to the local APIC on the processor during power up. This field was introduced in the Pentium 4 processor. Also, the code in init_intel was a bit overdesigned. Until Intel releases a chip with a non-power-of-2 sibling count on it, there's no point in all that bit bashing. arch/i386/kernel/cpu/intel.c | 25 +++++++------------------ 1 files changed, 7 insertions(+), 18 deletions(-) diff -puN arch/i386/kernel/cpu/intel.c~cpu_sibling_map-fix arch/i386/kernel/cpu/intel.c --- 25/arch/i386/kernel/cpu/intel.c~cpu_sibling_map-fix 2003-11-11 17:52:41.000000000 -0800 +++ 25-akpm/arch/i386/kernel/cpu/intel.c 2003-11-11 17:52:41.000000000 -0800 @@ -277,8 +277,6 @@ static void __init init_intel(struct cpu extern int phys_proc_id[NR_CPUS]; u32 eax, ebx, ecx, edx; - int index_lsb, index_msb, tmp; - int initial_apic_id; int cpu = smp_processor_id(); cpuid(1, &eax, &ebx, &ecx, &edx); @@ -287,8 +285,6 @@ static void __init init_intel(struct cpu if (smp_num_siblings == 1) { printk(KERN_INFO "CPU: Hyper-Threading is disabled\n"); } else if (smp_num_siblings > 1 ) { - index_lsb = 0; - index_msb = 31; /* * At this point we only support two siblings per * processor package. @@ -299,20 +295,13 @@ static void __init init_intel(struct cpu smp_num_siblings = 1; goto too_many_siblings; } - tmp = smp_num_siblings; - while ((tmp & 1) == 0) { - tmp >>=1 ; - index_lsb++; - } - tmp = smp_num_siblings; - while ((tmp & 0x80000000 ) == 0) { - tmp <<=1 ; - index_msb--; - } - if (index_lsb != index_msb ) - index_msb++; - initial_apic_id = ebx >> 24 & 0xff; - phys_proc_id[cpu] = initial_apic_id >> index_msb; + /* cpuid returns the value latched in the HW at reset, + * not the APIC ID register's value. For any box + * whose BIOS changes APIC IDs, like clustered APIC + * systems, we must use hard_smp_processor_id. + * See Intel's IA-32 SW Dev's Manual Vol2 under CPUID. + */ + phys_proc_id[cpu] = hard_smp_processor_id() & ~(smp_num_siblings - 1); printk(KERN_INFO "CPU: Physical Processor ID: %d\n", phys_proc_id[cpu]); _