x86/apic: Split disable_IO_APIC() into two functions to fix CONFIG_KEXEC_JUMP=y

Split  following patches disable_IO_APIC() will be broken up into
clear_IO_APIC() and restore_boot_irq_mode().

These two functions will be called separately where they are needed
to fix a regression introduced by:

  522e66464467 ("x86/apic: Disable I/O APIC before shutdown of the local APIC").

While the CONFIG_KEXEC_JUMP=y code doesn't call lapic_shutdown() before jump
like kexec/kdump, so it's not impacted by commit 522e66464467.

Hence here change clear_IO_APIC() as public, and replace disable_IO_APIC()
with clear_IO_APIC() and restore_boot_irq_mode() to keep CONFIG_KEXEC_JUMP=y
code unchanged in essence. No functional change.

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/20180214054656.3780-3-bhe@redhat.com
[ Rewrote the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/io_apic.h

@@ -193,6 +193,7 @@ static inline unsigned int io_apic_read(unsigned int apic, unsigned int reg)
 extern void setup_IO_APIC(void);
 extern void enable_IO_APIC(void);
 extern void disable_IO_APIC(void);
+extern void clear_IO_APIC(void);
 extern void restore_boot_irq_mode(void);
 extern int IO_APIC_get_PCI_irq_vector(int bus, int devfn, int pin);
 extern void print_IO_APICs(void);

arch/x86/kernel/apic/io_apic.c

@@ -587,7 +587,7 @@ static void clear_IO_APIC_pin(unsigned int apic, unsigned int pin)
 		       mpc_ioapic_id(apic), pin);
 }
 
-static void clear_IO_APIC (void)
+void clear_IO_APIC (void)
 {
 	int apic, pin;
 

arch/x86/kernel/machine_kexec_32.c

@@ -199,7 +199,8 @@ void machine_kexec(struct kimage *image)
 		 * one form or other. kexec jump path also need
 		 * one.
 		 */
-		disable_IO_APIC();
+		clear_IO_APIC();
+		restore_boot_irq_mode();
 #endif
 	}
 

arch/x86/kernel/machine_kexec_64.c

@@ -297,7 +297,8 @@ void machine_kexec(struct kimage *image)
 		 * one form or other. kexec jump path also need
 		 * one.
 		 */
-		disable_IO_APIC();
+		clear_IO_APIC();
+		restore_boot_irq_mode();
 #endif
 	}