KVM: track pid for VCPU only on KVM_RUN ioctl

We currently track the pid of the task that runs the VCPU in vcpu_load.
If a yield to that VCPU is triggered while the PID of the wrong thread
is active, the wrong thread might receive a yield, but this will most
likely not help the executing thread at all.  Instead, if we only track
the pid on the KVM_RUN ioctl, there are two possibilities:

1) the thread that did a non-KVM_RUN ioctl is holding a mutex that
the VCPU thread is waiting for.  In this case, the VCPU thread is not
runnable, but we also do not do a wrong yield.

2) the thread that did a non-KVM_RUN ioctl is sleeping, or doing
something that does not block the VCPU thread.  In this case, the
VCPU thread can receive the directed yield correctly.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
CC: Rik van Riel <riel@redhat.com>
CC: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
CC: Michael Mueller <mimu@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -124,15 +124,6 @@ int vcpu_load(struct kvm_vcpu *vcpu)
 
 	if (mutex_lock_killable(&vcpu->mutex))
 		return -EINTR;
-	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);
-		if (oldpid)
-			synchronize_rcu();
-		put_pid(oldpid);
-	}
 	cpu = get_cpu();
 	preempt_notifier_register(&vcpu->preempt_notifier);
 	kvm_arch_vcpu_load(vcpu, cpu);
@@ -2050,6 +2041,15 @@ 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);
+			if (oldpid)
+				synchronize_rcu();
+			put_pid(oldpid);
+		}
 		r = kvm_arch_vcpu_ioctl_run(vcpu, vcpu->run);
 		trace_kvm_userspace_exit(vcpu->run->exit_reason, r);
 		break;