x86/apic: Add a single-target IPI function to the apic

We still fall back on the "send mask" versions if an apic definition
doesn't have the single-target version, but at least this allows the
(trivial) case for the common clustered x2apic case.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Travis <travis@sgi.com>
Cc: Daniel J Blueman <daniel@numascale.com>
Link: http://lkml.kernel.org/r/20151104220848.737120838@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/apic.h

@@ -303,6 +303,7 @@ struct apic {
 				      unsigned int *apicid);
 
 	/* ipi */
+	void (*send_IPI)(int cpu, int vector);
 	void (*send_IPI_mask)(const struct cpumask *mask, int vector);
 	void (*send_IPI_mask_allbutself)(const struct cpumask *mask,
 					 int vector);

arch/x86/kernel/smp.c

@@ -114,6 +114,18 @@
 static atomic_t stopping_cpu = ATOMIC_INIT(-1);
 static bool smp_no_nmi_ipi = false;
 
+/*
+ * Helper wrapper: not all apic definitions support sending to
+ * a single CPU, so we fall back to sending to a mask.
+ */
+static void send_IPI_cpu(int cpu, int vector)
+{
+	if (apic->send_IPI)
+		apic->send_IPI(cpu, vector);
+	else
+		apic->send_IPI_mask(cpumask_of(cpu), vector);
+}
+
 /*
  * this function sends a 'reschedule' IPI to another CPU.
  * it goes straight through and wastes no time serializing
@@ -125,12 +137,12 @@ static void native_smp_send_reschedule(int cpu)
 		WARN_ON(1);
 		return;
 	}
-	apic->send_IPI_mask(cpumask_of(cpu), RESCHEDULE_VECTOR);
+	send_IPI_cpu(cpu, RESCHEDULE_VECTOR);
 }
 
 void native_send_call_func_single_ipi(int cpu)
 {
-	apic->send_IPI_mask(cpumask_of(cpu), CALL_FUNCTION_SINGLE_VECTOR);
+	send_IPI_cpu(cpu, CALL_FUNCTION_SINGLE_VECTOR);
 }
 
 void native_send_call_func_ipi(const struct cpumask *mask)