x86/apic: Install an empty physflat_init_apic_ldr

As the comment already stated, there is no need for setting up LDR (and
DFR) in physflat mode as it remains unused (see SDM, 10.6.2.1).
flat_init_apic_ldr only served as a placeholder for a nop operation so
far, causing no harm.

That will change when running over the Jailhouse hypervisor. Here we
must not touch LDR in a way that destroys the mapping originally set up
by the Linux root cell. Jailhouse enforces this setting in order to
efficiently validate any IPI requests sent by a cell.

Avoid a needless clash caused by flat_init_apic_ldr by installing a true
nop handler.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: jailhouse-dev@googlegroups.com
Link: https://lkml.kernel.org/r/f9867d294cdae4d45ed89d3a2e6adb524f4f6794.1511770314.git.jan.kiszka@siemens.com

arch/x86/kernel/apic/apic_flat_64.c

@@ -218,6 +218,15 @@ static int physflat_acpi_madt_oem_check(char *oem_id, char *oem_table_id)
 	return 0;
 }
 
+static void physflat_init_apic_ldr(void)
+{
+	/*
+	 * LDR and DFR are not involved in physflat mode, rather:
+	 * "In physical destination mode, the destination processor is
+	 * specified by its local APIC ID [...]." (Intel SDM, 10.6.2.1)
+	 */
+}
+
 static void physflat_send_IPI_allbutself(int vector)
 {
 	default_send_IPI_mask_allbutself_phys(cpu_online_mask, vector);
@@ -251,8 +260,7 @@ static struct apic apic_physflat __ro_after_init = {
 	.dest_logical			= 0,
 	.check_apicid_used		= NULL,
 
-	/* not needed, but shouldn't hurt: */
-	.init_apic_ldr			= flat_init_apic_ldr,
+	.init_apic_ldr			= physflat_init_apic_ldr,
 
 	.ioapic_phys_id_map		= NULL,
 	.setup_apic_routing		= NULL,