KVM: x86: do not retain disabled or invalid pvclock address

[Marcelo Tosatti <mtosatti@redhat.com>]

In case an invalid or disabled gpa is written to the SYSTEM_TIME
MSR, do not retain its value. This is not documented behaviour, 
nor should be supported.

Also clear it on system reset. Not doing so can hide bugs.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e00050c..ed4bfb7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1528,6 +1528,7 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 
 static void kvmclock_reset(struct kvm_vcpu *vcpu)
 {
+	vcpu->arch.time = 0;
 	if (vcpu->arch.time_page) {
 		kvm_release_page_dirty(vcpu->arch.time_page);
 		vcpu->arch.time_page = NULL;
@@ -1632,8 +1633,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
 
 		/* we verify if the enable bit is set... */
-		if (!(data & 1))
+		if (!(data & 1)) {
+			vcpu->arch.time = 0;
 			break;
+		}
 
 		/* ...but clean it before doing the actual write */
 		vcpu->arch.time_offset = data & ~(PAGE_MASK | 1);
@@ -1641,8 +1644,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 		vcpu->arch.time_page =
 				gfn_to_page(vcpu->kvm, data >> PAGE_SHIFT);
 
-		if (is_error_page(vcpu->arch.time_page))
+		if (is_error_page(vcpu->arch.time_page)) {
 			vcpu->arch.time_page = NULL;
+			vcpu->arch.time = 0;
+		}
 
 		break;
 	}