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>

1 files changed, 5 insertions, 2 deletions

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 4abf08aab3d4..8a0215042a5f 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/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);