x86/apic: Set up through-local-APIC mode on the boot CPU if 'noapic' specified

Currently the kdump kernel becomes very slow if 'noapic' is specified.
Normal kernel doesn't have this bug.

Kernel parameter 'noapic' is used to disable IO-APIC in system for
testing or special purpose. Here the root cause is that in kdump
kernel LAPIC is disabled since commit:

  522e6646 ("x86/apic: Disable I/O APIC before shutdown of the local APIC")

In this case we need set up through-local-APIC on boot CPU in
setup_local_APIC().

In normal kernel the legacy irq mode is enabled by the BIOS. If
it is virtual wire mode, the local-APIC has been enabled and set as
through-local-APIC.

Though we fixed the regression introduced by commit 522e6646,
to further improve robustness set up the through-local-APIC mode
explicitly, do not rely on the default boot IRQ mode.
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: douly.fnst@cn.fujitsu.com
Cc: joro@8bytes.org
Cc: prarit@redhat.com
Cc: uobergfe@redhat.com
Link: http://lkml.kernel.org/r/20180214054656.3780-7-bhe@redhat.com
[ Rewrote the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/apic/apic.c

@@ -1570,7 +1570,7 @@ static void setup_local_APIC(void)
 	 * TODO: set up through-local-APIC from through-I/O-APIC? --macro
 	 */
 	value = apic_read(APIC_LVT0) & APIC_LVT_MASKED;
-	if (!cpu && (pic_mode || !value)) {
+	if (!cpu && (pic_mode || !value || skip_ioapic_setup)) {
 		value = APIC_DM_EXTINT;
 		apic_printk(APIC_VERBOSE, "enabled ExtINT on CPU#%d\n", cpu);
 	} else {