KVM: Remove deprecated create_singlethread_workqueue

The workqueue "irqfd_cleanup_wq" queues a single work item
&irqfd->shutdown and hence doesn't require ordering. It is a host-wide
workqueue for issuing deferred shutdown requests aggregated from all
vm* instances. It is not being used on a memory reclaim path.
Hence, it has been converted to use system_wq.
The work item has been flushed in kvm_irqfd_release().

The workqueue "wqueue" queues a single work item &timer->expired
and hence doesn't require ordering. Also, it is not being used on
a memory reclaim path. Hence, it has been converted to use system_wq.

System workqueues have been able to handle high level of concurrency
for a long time now and hence it's not required to have a singlethreaded
workqueue just to gain concurrency. Unlike a dedicated per-cpu workqueue
created with create_singlethread_workqueue(), system_wq allows multiple
work items to overlap executions even on the same CPU; however, a
per-cpu workqueue doesn't have any CPU locality or global ordering
guarantee unless the target CPU is explicitly specified and thus the
increase of local concurrency shouldn't make any difference.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/arm/arch_timer.c

@@ -31,7 +31,6 @@
 #include "trace.h"
 
 static struct timecounter *timecounter;
-static struct workqueue_struct *wqueue;
 static unsigned int host_vtimer_irq;
 static u32 host_vtimer_irq_flags;
 
@@ -141,7 +140,7 @@ static enum hrtimer_restart kvm_timer_expire(struct hrtimer *hrt)
 		return HRTIMER_RESTART;
 	}
 
-	queue_work(wqueue, &timer->expired);
+	schedule_work(&timer->expired);
 	return HRTIMER_NORESTART;
 }
 
@@ -449,12 +448,6 @@ int kvm_timer_hyp_init(void)
 		goto out;
 	}
 
-	wqueue = create_singlethread_workqueue("kvm_arch_timer");
-	if (!wqueue) {
-		err = -ENOMEM;
-		goto out_free;
-	}
-
 	kvm_info("virtual timer IRQ%d\n", host_vtimer_irq);
 
 	cpuhp_setup_state(CPUHP_AP_KVM_ARM_TIMER_STARTING,
@@ -518,7 +511,7 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
 	 * VCPUs have the enabled variable set, before entering the guest, if
 	 * the arch timers are enabled.
 	 */
-	if (timecounter && wqueue)
+	if (timecounter)
 		timer->enabled = 1;
 
 	return 0;