kvm: Record the preemption status of vcpus using preempt notifiers

Note that we mark as preempted only when vcpu's task state was
Running during preemption.

Thanks Jiannan, Avi for preemption notifier ideas. Thanks Gleb, PeterZ
for their precious suggestions. Thanks Srikar for an idea on avoiding
rcu lock while checking task state that improved overcommit numbers.
Reviewed-by: Chegu Vinod <chegu_vinod@hp.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

include/linux/kvm_host.h

@@ -253,6 +253,7 @@ struct kvm_vcpu {
 		bool dy_eligible;
 	} spin_loop;
 #endif
+	bool preempted;
 	struct kvm_vcpu_arch arch;
 };
 

virt/kvm/kvm_main.c

@@ -244,6 +244,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
 
 	kvm_vcpu_set_in_spin_loop(vcpu, false);
 	kvm_vcpu_set_dy_eligible(vcpu, false);
+	vcpu->preempted = false;
 
 	r = kvm_arch_vcpu_init(vcpu);
 	if (r < 0)
@@ -2880,6 +2881,8 @@ struct kvm_vcpu *preempt_notifier_to_vcpu(struct preempt_notifier *pn)
 static void kvm_sched_in(struct preempt_notifier *pn, int cpu)
 {
 	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
+	if (vcpu->preempted)
+		vcpu->preempted = false;
 
 	kvm_arch_vcpu_load(vcpu, cpu);
 }
@@ -2889,6 +2892,8 @@ static void kvm_sched_out(struct preempt_notifier *pn,
 {
 	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
 
+	if (current->state == TASK_RUNNING)
+		vcpu->preempted = true;
 	kvm_arch_vcpu_put(vcpu);
 }