s390/smp: implement arch_irq_work_raise()

The immediate need to have this is to have bpf_send_signal() send the
signal ASAP instead of during the next hrtimer interrupt. However, it
should also improve irq_work_queue() latencies in general, as well as
get s390 out of the lame architectures list [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/irq_work.c?h=v5.11#n45

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/irq_work.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_S390_IRQ_WORK_H
+#define _ASM_S390_IRQ_WORK_H
+
+static inline bool arch_irq_work_has_interrupt(void)
+{
+	return true;
+}
+
+void arch_irq_work_raise(void);
+
+#endif /* _ASM_S390_IRQ_WORK_H */

arch/s390/kernel/smp.c

@@ -30,6 +30,7 @@
 #include <linux/delay.h>
 #include <linux/interrupt.h>
 #include <linux/irqflags.h>
+#include <linux/irq_work.h>
 #include <linux/cpu.h>
 #include <linux/slab.h>
 #include <linux/sched/hotplug.h>
@@ -62,6 +63,7 @@ enum {
 	ec_call_function_single,
 	ec_stop_cpu,
 	ec_mcck_pending,
+	ec_irq_work,
 };
 
 enum {
@@ -508,6 +510,8 @@ static void smp_handle_ext_call(void)
 		generic_smp_call_function_single_interrupt();
 	if (test_bit(ec_mcck_pending, &bits))
 		__s390_handle_mcck();
+	if (test_bit(ec_irq_work, &bits))
+		irq_work_run();
 }
 
 static void do_ext_call_interrupt(struct ext_code ext_code,
@@ -540,6 +544,13 @@ void smp_send_reschedule(int cpu)
 	pcpu_ec_call(pcpu_devices + cpu, ec_schedule);
 }
 
+#ifdef CONFIG_IRQ_WORK
+void arch_irq_work_raise(void)
+{
+	pcpu_ec_call(pcpu_devices + smp_processor_id(), ec_irq_work);
+}
+#endif
+
 /*
  * parameter area for the set/clear control bit callbacks
  */