[RFC -v6 PATCH 7/8] kvm: keep track of which task is running a KVM vcpu

[Rik van Riel <riel@redhat.com>]

Keep track of which task is running a KVM vcpu.  This helps us
figure out later what task to wake up if we want to boost a
vcpu that got preempted.

Unfortunately there are no guarantees that the same task
always keeps the same vcpu, so we can only track the task
across a single "run" of the vcpu.

Signed-off-by: Rik van Riel <riel@redhat.com>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index a055742..9d56ed5 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -81,6 +81,7 @@ struct kvm_vcpu {
 #endif
 	int vcpu_id;
 	struct mutex mutex;
+	struct pid *pid;
 	int   cpu;
 	atomic_t guest_mode;
 	struct kvm_run *run;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 5225052..86c4905 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -185,6 +185,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
 	vcpu->cpu = -1;
 	vcpu->kvm = kvm;
 	vcpu->vcpu_id = id;
+	vcpu->pid = NULL;
 	init_waitqueue_head(&vcpu->wq);
 
 	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
@@ -208,6 +209,8 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_init);
 
 void kvm_vcpu_uninit(struct kvm_vcpu *vcpu)
 {
+	if (vcpu->pid)
+		put_pid(vcpu->pid);
 	kvm_arch_vcpu_uninit(vcpu);
 	free_page((unsigned long)vcpu->run);
 }
@@ -1456,6 +1459,14 @@ static long kvm_vcpu_ioctl(struct file *filp,
 		r = -EINVAL;
 		if (arg)
 			goto out;
+		if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) {
+			/* The thread running this VCPU changed. */
+			struct pid *oldpid = vcpu->pid;
+			struct pid *newpid = get_task_pid(current, PIDTYPE_PID);
+			rcu_assign_pointer(vcpu->pid, newpid);
+			synchronize_rcu();
+			put_pid(oldpid);
+		}
 		r = kvm_arch_vcpu_ioctl_run(vcpu, vcpu->run);
 		break;
 	case KVM_GET_REGS: {