s390/wti: Prepare graceful CPU pre-emption on wti reception

When a warning track interrupt is received, the kernel has only a very
limited amount of time to make sure, that the CPU can be yielded as
gracefully as possible before being pre-empted by the hypervisor.

The interrupt handler for the wti therefore unparks a kernel thread
which has being created on boot re-using the CPU hotplug kernel thread
infrastructure. These threads exist per CPU and are assigned the
highest possible real-time priority. This makes sure, that said threads
will execute as soon as possible as the scheduler should pre-empt any
other running user tasks to run the real-time thread.

Furthermore, the interrupt handler disables all I/O interrupts to
prevent additional interrupt processing on the soon-preempted CPU.
Interrupt handlers are likely to take kernel locks, which in the worst
case, will be kept while the interrupt handler is pre-empted from itself
underlying physical CPU. In that case, all tasks or interrupt handlers
on other CPUs would have to wait for the pre-empted CPU being dispatched
again. By preventing further interrupt processing, this risk is
minimized.

Once the CPU gets dispatched again, the real-time kernel thread regains
control, reenables interrupts and parks itself again.

Acked-by: Heiko Carstens <[e-mail]>
Reviewed-by: Mete Durlu <[e-mail]>
Signed-off-by: Tobias Huschle <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>

arch/s390/kernel/Makefile

···
 obj-y	+= runtime_instr.o cache.o fpu.o dumpstack.o guarded_storage.o sthyi.o
 obj-y	+= entry.o reipl.o kdebugfs.o alternative.o
 obj-y	+= nospec-branch.o ipl_vmparm.o machine_kexec_reloc.o unwind_bc.o
-obj-y	+= smp.o text_amode31.o stacktrace.o abs_lowcore.o facility.o uv.o
+obj-y	+= smp.o text_amode31.o stacktrace.o abs_lowcore.o facility.o uv.o wti.o
 
 extra-y				+= vmlinux.lds
 

arch/s390/kernel/wti.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Support for warning track interruption
+ *
+ * Copyright IBM Corp. 2023
+ */
+
+#include <linux/smpboot.h>
+#include <linux/irq.h>
+#include <uapi/linux/sched/types.h>
+#include <asm/diag.h>
+#include <asm/sclp.h>
+
+struct wti_state {
+	/*
+	 * Represents the real-time thread responsible to
+	 * acknowledge the warning-track interrupt and trigger
+	 * preliminary and postliminary precautions.
+	 */
+	struct task_struct	*thread;
+	/*
+	 * If pending is true, the real-time thread must be scheduled.
+	 * If not, a wake up of that thread will remain a noop.
+	 */
+	bool			pending;
+};
+
+static DEFINE_PER_CPU(struct wti_state, wti_state);
+
+/*
+ * During a warning-track grace period, interrupts are disabled
+ * to prevent delays of the warning-track acknowledgment.
+ *
+ * Once the CPU is physically dispatched again, interrupts are
+ * re-enabled.
+ */
+
+static void wti_irq_disable(void)
+{
+	unsigned long flags;
+	struct ctlreg cr6;
+
+	local_irq_save(flags);
+	local_ctl_store(6, &cr6);
+	/* disable all I/O interrupts */
+	cr6.val &= ~0xff000000UL;
+	local_ctl_load(6, &cr6);
+	local_irq_restore(flags);
+}
+
+static void wti_irq_enable(void)
+{
+	unsigned long flags;
+	struct ctlreg cr6;
+
+	local_irq_save(flags);
+	local_ctl_store(6, &cr6);
+	/* enable all I/O interrupts */
+	cr6.val |= 0xff000000UL;
+	local_ctl_load(6, &cr6);
+	local_irq_restore(flags);
+}
+
+static void wti_interrupt(struct ext_code ext_code,
+			  unsigned int param32, unsigned long param64)
+{
+	struct wti_state *st = this_cpu_ptr(&wti_state);
+
+	inc_irq_stat(IRQEXT_WTI);
+	wti_irq_disable();
+	st->pending = true;
+	wake_up_process(st->thread);
+}
+
+static int wti_pending(unsigned int cpu)
+{
+	struct wti_state *st = per_cpu_ptr(&wti_state, cpu);
+
+	return st->pending;
+}
+
+static void wti_thread_fn(unsigned int cpu)
+{
+	struct wti_state *st = per_cpu_ptr(&wti_state, cpu);
+
+	st->pending = false;
+	/*
+	 * Yield CPU voluntarily to the hypervisor. Control
+	 * resumes when hypervisor decides to dispatch CPU
+	 * to this LPAR again.
+	 */
+	diag49c(DIAG49C_SUBC_ACK);
+	wti_irq_enable();
+}
+
+static struct smp_hotplug_thread wti_threads = {
+	.store			= &wti_state.thread,
+	.thread_should_run	= wti_pending,
+	.thread_fn		= wti_thread_fn,
+	.thread_comm		= "cpuwti/%u",
+	.selfparking		= false,
+};
+
+static int __init wti_init(void)
+{
+	struct sched_param wti_sched_param = { .sched_priority = MAX_RT_PRIO - 1 };
+	struct wti_state *st;
+	int cpu, rc;
+
+	rc = -EOPNOTSUPP;
+	if (!sclp.has_wti)
+		goto out;
+	rc = smpboot_register_percpu_thread(&wti_threads);
+	if (WARN_ON(rc))
+		goto out;
+	for_each_online_cpu(cpu) {
+		st = per_cpu_ptr(&wti_state, cpu);
+		sched_setscheduler(st->thread, SCHED_FIFO, &wti_sched_param);
+	}
+	rc = register_external_irq(EXT_IRQ_WARNING_TRACK, wti_interrupt);
+	if (rc) {
+		pr_warn("Couldn't request external interrupt 0x1007\n");
+		goto out_thread;
+	}
+	irq_subclass_register(IRQ_SUBCLASS_WARNING_TRACK);
+	rc = diag49c(DIAG49C_SUBC_REG);
+	if (rc) {
+		pr_warn("Failed to register warning track interrupt through DIAG 49C\n");
+		rc = -EOPNOTSUPP;
+		goto out_subclass;
+	}
+	goto out;
+out_subclass:
+	irq_subclass_unregister(IRQ_SUBCLASS_WARNING_TRACK);
+	unregister_external_irq(EXT_IRQ_WARNING_TRACK, wti_interrupt);
+out_thread:
+	smpboot_unregister_percpu_thread(&wti_threads);
+out:
+	return rc;
+}
+late_initcall(wti_init);