KVM: x86: retain pvclock guest stopped bit in guest memory

Otherwise its possible for an unrelated KVM_REQ_UPDATE_CLOCK (such as due to CPU
migration) to clear the bit.

Noticed by Paolo Bonzini.

Reviewed-by: Gleb Natapov <[email protected]>
Reviewed-by: Glauber Costa <[email protected]>
Signed-off-by: Marcelo Tosatti <[email protected]>

arch/x86/kvm/x86.c

@@ -1145,6 +1145,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	unsigned long this_tsc_khz;
 	s64 kernel_ns, max_kernel_ns;
 	u64 tsc_timestamp;
+	struct pvclock_vcpu_time_info *guest_hv_clock;
 	u8 pvclock_flags;
 
 	/* Keep irq disabled to prevent changes to the clock */
@@ -1228,13 +1229,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	vcpu->last_kernel_ns = kernel_ns;
 	vcpu->last_guest_tsc = tsc_timestamp;
 
-	pvclock_flags = 0;
-	if (vcpu->pvclock_set_guest_stopped_request) {
-		pvclock_flags |= PVCLOCK_GUEST_STOPPED;
-		vcpu->pvclock_set_guest_stopped_request = false;
-	}
-
-	vcpu->hv_clock.flags = pvclock_flags;
 
 	/*
 	 * The interface expects us to write an even number signaling that the
@@ -1245,6 +1239,18 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 
 	shared_kaddr = kmap_atomic(vcpu->time_page);
 
+	guest_hv_clock = shared_kaddr + vcpu->time_offset;
+
+	/* retain PVCLOCK_GUEST_STOPPED if set in guest copy */
+	pvclock_flags = (guest_hv_clock->flags & PVCLOCK_GUEST_STOPPED);
+
+	if (vcpu->pvclock_set_guest_stopped_request) {
+		pvclock_flags |= PVCLOCK_GUEST_STOPPED;
+		vcpu->pvclock_set_guest_stopped_request = false;
+	}
+
+	vcpu->hv_clock.flags = pvclock_flags;
+
 	memcpy(shared_kaddr + vcpu->time_offset, &vcpu->hv_clock,
 	       sizeof(vcpu->hv_clock));