KVM: x86: Run PIT work in own kthread

We can't run PIT IRQ injection work in the interrupt context of the host
timer. This would allow the user to influence the handler complexity by
asking for a broadcast to a large number of VCPUs. Therefore, this work
was pushed into workqueue context in 9d244caf2e. However, this prevents
prioritizing the PIT injection over other task as workqueues share
kernel threads.

This replaces the workqueue with a kthread worker and gives that thread
a name in the format "kvm-pit/<owner-process-pid>". That allows to
identify and adjust the kthread priority according to the VM process
parameters.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -1810,6 +1810,14 @@ Valid flags are:
 
 #define KVM_PIT_SPEAKER_DUMMY     1 /* emulate speaker port stub */
 
+PIT timer interrupts may use a per-VM kernel thread for injection. If it
+exists, this thread will have a name of the following pattern:
+
+kvm-pit/<owner-process-pid>
+
+When running a guest with elevated priorities, the scheduling parameters of
+this thread may have to be adjusted accordingly.
+
 This IOCTL replaces the obsolete KVM_CREATE_PIT.
 
 

arch/x86/kvm/i8254.c

@@ -34,7 +34,6 @@
 
 #include <linux/kvm_host.h>
 #include <linux/slab.h>
-#include <linux/workqueue.h>
 
 #include "irq.h"
 #include "i8254.h"
@@ -249,7 +248,7 @@ static void kvm_pit_ack_irq(struct kvm_irq_ack_notifier *kian)
 		/* in this case, we had multiple outstanding pit interrupts
 		 * that we needed to inject.  Reinject
 		 */
-		queue_work(ps->pit->wq, &ps->pit->expired);
+		queue_kthread_work(&ps->pit->worker, &ps->pit->expired);
 	ps->irq_ack = 1;
 	spin_unlock(&ps->inject_lock);
 }
@@ -270,7 +269,7 @@ void __kvm_migrate_pit_timer(struct kvm_vcpu *vcpu)
 static void destroy_pit_timer(struct kvm_pit *pit)
 {
 	hrtimer_cancel(&pit->pit_state.pit_timer.timer);
-	cancel_work_sync(&pit->expired);
+	flush_kthread_work(&pit->expired);
 }
 
 static bool kpit_is_periodic(struct kvm_timer *ktimer)
@@ -284,7 +283,7 @@ static struct kvm_timer_ops kpit_ops = {
 	.is_periodic = kpit_is_periodic,
 };
 
-static void pit_do_work(struct work_struct *work)
+static void pit_do_work(struct kthread_work *work)
 {
 	struct kvm_pit *pit = container_of(work, struct kvm_pit, expired);
 	struct kvm *kvm = pit->kvm;
@@ -328,7 +327,7 @@ static enum hrtimer_restart pit_timer_fn(struct hrtimer *data)
 
 	if (ktimer->reinject || !atomic_read(&ktimer->pending)) {
 		atomic_inc(&ktimer->pending);
-		queue_work(pt->wq, &pt->expired);
+		queue_kthread_work(&pt->worker, &pt->expired);
 	}
 
 	if (ktimer->t_ops->is_periodic(ktimer)) {
@@ -353,7 +352,7 @@ static void create_pit_timer(struct kvm *kvm, u32 val, int is_period)
 
 	/* TODO The new value only affected after the retriggered */
 	hrtimer_cancel(&pt->timer);
-	cancel_work_sync(&ps->pit->expired);
+	flush_kthread_work(&ps->pit->expired);
 	pt->period = interval;
 	ps->is_periodic = is_period;
 
@@ -669,6 +668,8 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
 {
 	struct kvm_pit *pit;
 	struct kvm_kpit_state *pit_state;
+	struct pid *pid;
+	pid_t pid_nr;
 	int ret;
 
 	pit = kzalloc(sizeof(struct kvm_pit), GFP_KERNEL);
@@ -685,14 +686,20 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
 	mutex_lock(&pit->pit_state.lock);
 	spin_lock_init(&pit->pit_state.inject_lock);
 
-	pit->wq = create_singlethread_workqueue("kvm-pit-wq");
-	if (!pit->wq) {
+	pid = get_pid(task_tgid(current));
+	pid_nr = pid_vnr(pid);
+	put_pid(pid);
+
+	init_kthread_worker(&pit->worker);
+	pit->worker_task = kthread_run(kthread_worker_fn, &pit->worker,
+				       "kvm-pit/%d", pid_nr);
+	if (IS_ERR(pit->worker_task)) {
 		mutex_unlock(&pit->pit_state.lock);
 		kvm_free_irq_source_id(kvm, pit->irq_source_id);
 		kfree(pit);
 		return NULL;
 	}
-	INIT_WORK(&pit->expired, pit_do_work);
+	init_kthread_work(&pit->expired, pit_do_work);
 
 	kvm->arch.vpit = pit;
 	pit->kvm = kvm;
@@ -736,7 +743,7 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
 	kvm_unregister_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
 	kvm_unregister_irq_ack_notifier(kvm, &pit_state->irq_ack_notifier);
 	kvm_free_irq_source_id(kvm, pit->irq_source_id);
-	destroy_workqueue(pit->wq);
+	kthread_stop(pit->worker_task);
 	kfree(pit);
 	return NULL;
 }
@@ -756,10 +763,10 @@ void kvm_free_pit(struct kvm *kvm)
 		mutex_lock(&kvm->arch.vpit->pit_state.lock);
 		timer = &kvm->arch.vpit->pit_state.pit_timer.timer;
 		hrtimer_cancel(timer);
-		cancel_work_sync(&kvm->arch.vpit->expired);
+		flush_kthread_work(&kvm->arch.vpit->expired);
+		kthread_stop(kvm->arch.vpit->worker_task);
 		kvm_free_irq_source_id(kvm, kvm->arch.vpit->irq_source_id);
 		mutex_unlock(&kvm->arch.vpit->pit_state.lock);
-		destroy_workqueue(kvm->arch.vpit->wq);
 		kfree(kvm->arch.vpit);
 	}
 }

arch/x86/kvm/i8254.h

 #ifndef __I8254_H
 #define __I8254_H
 
+#include <linux/kthread.h>
+
 #include "iodev.h"
 
 struct kvm_kpit_channel_state {
@@ -39,8 +41,9 @@ struct kvm_pit {
 	struct kvm_kpit_state pit_state;
 	int irq_source_id;
 	struct kvm_irq_mask_notifier mask_notifier;
-	struct workqueue_struct *wq;
-	struct work_struct expired;
+	struct kthread_worker worker;
+	struct task_struct *worker_task;
+	struct kthread_work expired;
 };
 
 #define KVM_PIT_BASE_ADDRESS	    0x40