x86/apic: Soft disable APIC before initializing it

If the APIC was already enabled on entry of setup_local_APIC() then
disabling it soft via the SPIV register makes a lot of sense.

That masks all LVT entries and brings it into a well defined state.

Otherwise previously enabled LVTs which are not touched in the setup
function stay unmasked and might surprise the just booting kernel.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190722105219.068290579@linutronix.de

arch/x86/kernel/apic/apic.c

@@ -1522,6 +1522,14 @@ static void setup_local_APIC(void)
 		return;
 	}
 
+	/*
+	 * If this comes from kexec/kcrash the APIC might be enabled in
+	 * SPIV. Soft disable it before doing further initialization.
+	 */
+	value = apic_read(APIC_SPIV);
+	value &= ~APIC_SPIV_APIC_ENABLED;
+	apic_write(APIC_SPIV, value);
+
 #ifdef CONFIG_X86_32
 	/* Pound the ESR really hard over the head with a big hammer - mbligh */
 	if (lapic_is_integrated() && apic->disable_esr) {