KVM: x86: move steal time initialization to vcpu entry time

commit 7cae2bed upstream.

As reported at https://bugs.launchpad.net/qemu/+bug/1494350,
it is possible to have vcpu->arch.st.last_steal initialized
from a thread other than vcpu thread, say the iothread, via
KVM_SET_MSRS.

Which can cause an overflow later (when subtracting from vcpu threads
sched_info.run_delay).

To avoid that, move steal time accumulation to vcpu entry time,
before copying steal time data to guest.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Liang Chen <liangchen.linux@gmail.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

arch/x86/kvm/x86.c

@@ -2025,6 +2025,8 @@ static void accumulate_steal_time(struct kvm_vcpu *vcpu)
 
 static void record_steal_time(struct kvm_vcpu *vcpu)
 {
+	accumulate_steal_time(vcpu);
+
 	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
 		return;
 
@@ -2157,12 +2159,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (!(data & KVM_MSR_ENABLED))
 			break;
 
-		vcpu->arch.st.last_steal = current->sched_info.run_delay;
-
-		preempt_disable();
-		accumulate_steal_time(vcpu);
-		preempt_enable();
-
 		kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 
 		break;
@@ -2859,7 +2855,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		vcpu->cpu = cpu;
 	}
 
-	accumulate_steal_time(vcpu);
 	kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 }