x86, apic: explain the purpose of max_physical_apicid

#include <asm/smp.h>

unsigned int num_processors;

unsigned disabled_cpus __cpuinitdata;

/* Processor that is doing the boot up */

unsigned int boot_cpu_physical_apicid = -1U;

EXPORT_SYMBOL(boot_cpu_physical_apicid);

/*

 * The highest APIC ID seen during enumeration.

 *

 * This determines the messaging protocol we can use: if all APIC IDs

 * are in the 0 ... 7 range, then we can use logical addressing which

 * has some performance advantages (better broadcasting).

 *

 * If there's an APIC ID above 8, we use physical addressing.

 */

unsigned int max_physical_apicid;

/* Bitmask of physically existing CPUs */

/*

 * Bitmask of physically existing CPUs:

 */

physid_mask_t phys_cpu_present_map;

/*