[PATCH] x86_64: use TSC on SMP EM64T machines

x86-64 MP kernels always used HPET timing when available.  This was done
because AMD systems don't have synchronized TSCs in all cases.  On Intel
this is not true (except Summit), so use faster TSC timing for those.

Also only enable HPET gettimeofday when there is more than one CPU online.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/time.c

@@ -909,15 +909,28 @@ void __init time_init_smp(void)
 {
 	char *timetype;
 
-	if (vxtime.hpet_address) {
+	/*
+	 * AMD systems with more than one CPU don't have fully synchronized
+	 * TSCs. Always use HPET gettimeofday for these, although it is slower.
+	 * Intel SMP systems usually have synchronized TSCs, so use always
+	 * the TSC.
+	 *
+	 * Exceptions:
+	 * IBM Summit. Will need to be special cased later.
+ 	 * AMD dual core may also not need HPET. Check me.
+	 */
+	if (vxtime.hpet_address &&
+	    num_online_cpus() > 1 &&
+	    boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
 		timetype = "HPET";
 		vxtime.last = hpet_readl(HPET_T0_CMP) - hpet_tick;
 		vxtime.mode = VXTIME_HPET;
 		do_gettimeoffset = do_gettimeoffset_hpet;
 	} else {
-		timetype = "PIT/TSC";
+		timetype = vxtime.hpet_address ? "HPET/TSC" : "PIT/TSC";
 		vxtime.mode = VXTIME_TSC;
 	}
+
 	printk(KERN_INFO "time.c: Using %s based timekeeping.\n", timetype);
 }