x86, ioapic: Avoid writing io_apic id if already correct

For 32bit mptable path, setup_ids_from_mpc() always writes the io_apic
id register, even there is no change needed.

Skip the write, when readout and mptable match.
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
LKML-Reference: <4CFDF785.7010401@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/apic/io_apic.c

@@ -2007,9 +2007,12 @@ void __init setup_ioapic_ids_from_mpc_nocheck(void)
 						= mp_ioapics[apic_id].apicid;
 
 		/*
-		 * Read the right value from the MPC table and
-		 * write it into the ID register.
+		 * Update the ID register according to the right value
+		 * from the MPC table if they are different.
 		 */
+		if (mp_ioapics[apic_id].apicid == reg_00.bits.ID)
+			continue;
+
 		apic_printk(APIC_VERBOSE, KERN_INFO
 			"...changing IO-APIC physical APIC ID to %d ...",
 			mp_ioapics[apic_id].apicid);