KVM: Fix wallclock version writing race

Commit Message

Avi Kivity May 4, 2010, 12:02 p.m. UTC

None

Patch

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f6f8dad..c3152d7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -754,14 +754,22 @@  static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 
 static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
 {
-	static int version;
+	int version;
+	int r;
 	struct pvclock_wall_clock wc;
 	struct timespec boot;
 
 	if (!wall_clock)
 		return;
 
-	version++;
+	r = kvm_read_guest(kvm, wall_clock, &version, sizeof(version));
+	if (r)
+		return;
+
+	if (version & 1)
+		++version;  /* first time write, random junk */
+
+	++version;
 
 	kvm_write_guest(kvm, wall_clock, &version, sizeof(version));