x86, apic: use logical flat for systems with <= 8 logical cpus

We can use logical flat mode if there are <= 8 logical cpu's
(irrespective of physical apic id values). This will enable simplified
and efficient IPI and device interrupt routing on such platforms.

This has been tested to work on both Intel and AMD platforms.
Exceptions like IBM summit platform which can't use logical flat mode
are addressed by using OEM platform checks.

Signed-off-by: Suresh Siddha <[e-mail]>
Signed-off-by: Yinghai Lu <[e-mail]>
Cc: Ananth N Mavinakayanahalli <[e-mail]>
Cc: Chris McDermott <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/kernel/apic/apic.c

···
 
 /*
  * 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;
 
···
 		max_physical_apicid = apicid;
 
 #ifdef CONFIG_X86_32
-	/*
-	 * Would be preferable to switch to bigsmp when CONFIG_HOTPLUG_CPU=y
-	 * but we need to work other dependencies like SMP_SUSPEND etc
-	 * before this can be done without some confusion.
-	 * if (CPU_HOTPLUG_ENABLED || num_processors > 8)
-	 *       - Ashok Raj <[e-mail]>
-	 */
-	if (max_physical_apicid >= 8) {
+	if (num_processors > 8) {
 		switch (boot_cpu_data.x86_vendor) {
 		case X86_VENDOR_INTEL:
 			if (!APIC_XAPIC(version)) {

arch/x86/kernel/apic/probe_64.c

···
 			apic = &apic_x2apic_phys;
 		else
 			apic = &apic_x2apic_cluster;
-		printk(KERN_INFO "Setting APIC routing to %s\n", apic->name);
 	}
 #endif
 
-	if (apic == &apic_flat) {
-		if (max_physical_apicid >= 8)
+	if (apic == &apic_flat && num_processors > 8)
 			apic = &apic_physflat;
-		printk(KERN_INFO "Setting APIC routing to %s\n", apic->name);
-	}
+
+	printk(KERN_INFO "Setting APIC routing to %s\n", apic->name);
 
 	if (is_vsmp_box()) {
 		/* need to update phys_pkg_id */