Currently kdump kernel becomes very slow if 'noapic' is specified.
Normal kernel won't.

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 522e664644
("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

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

Though we fix the regression introduced by criminal commit 522e664644,
for safety and clarity, better set up through-local-APIC explicitly,
but not rely on the default boot irq mode.

Do it now.

Signed-off-by: Baoquan He <>
 arch/x86/kernel/apic/apic.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 25ddf02598d2..3fc259b4dd2d 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1570,7 +1570,7 @@ 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 {

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