[PATCH 1/3] x86/apic: Set up through LAPIC on boot CPU's LINT0 if ioapic is disabled to kexec/kdump kernel

[Baoquan He]

Kdump kernel will become very slow if 'noapic' is specified in kernel
command line. Normal kernel doesn't have this issue.

This is because the legacy irq mode is disabled in crashed kernel before
jump jump to kdump kernel since commit 522e66464467 ("x86/apic: Disable I/O
APIC before shutdown of the local APIC") is merged. While in normal kernel,
the legacy irq mode has been set in BIOS.

So we need set the delivery mode AS ExtINT for LVT0 of boot CPU's LAPIC
explicitly if IO-APIC is disabled, to set up through-local-APIC.

Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
---
 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 880441f24146..7e613fb90630 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1521,7 +1521,7 @@ 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 {
-- 
2.5.5