diff --git a/xen/arch/x86/hvm/vpt.c b/xen/arch/x86/hvm/vpt.c
index 4c2afe2e9154..560fab9cfc60 100644
--- a/xen/arch/x86/hvm/vpt.c
+++ b/xen/arch/x86/hvm/vpt.c
@@ -153,32 +153,43 @@ static int pt_irq_masked(struct periodic_time *pt)
     return 1;
 }
 
+/*
+ * Functions which read (maybe write) all periodic_time instances
+ * attached to a particular vCPU use pt_vcpu_{un}lock locking helpers.
+ *
+ * Such users are explicitly forbidden from changing the value of the
+ * pt->vcpu field, because another thread holding the pt_migrate lock
+ * may already be spinning waiting for your vcpu lock.
+ */
 static void pt_vcpu_lock(struct vcpu *v)
 {
-    read_lock(&v->domain->arch.hvm.pl_time->pt_migrate);
     spin_lock(&v->arch.hvm.tm_lock);
 }
 
 static void pt_vcpu_unlock(struct vcpu *v)
 {
     spin_unlock(&v->arch.hvm.tm_lock);
-    read_unlock(&v->domain->arch.hvm.pl_time->pt_migrate);
 }
 
+/*
+ * Functions which want to modify a particular periodic_time object
+ * use pt_{un}lock locking helpers.
+ *
+ * These users lock whichever vCPU the periodic_time is attached to,
+ * but since the vCPU could be modified without holding any lock, they
+ * need to take an additional lock that protects against pt->vcpu
+ * changing.
+ */
 static void pt_lock(struct periodic_time *pt)
 {
-    /*
-     * We cannot use pt_vcpu_lock here, because we need to acquire the
-     * per-domain lock first and then (re-)fetch the value of pt->vcpu, or
-     * else we might be using a stale value of pt->vcpu.
-     */
     read_lock(&pt->vcpu->domain->arch.hvm.pl_time->pt_migrate);
     spin_lock(&pt->vcpu->arch.hvm.tm_lock);
 }
 
 static void pt_unlock(struct periodic_time *pt)
 {
-    pt_vcpu_unlock(pt->vcpu);
+    spin_unlock(&pt->vcpu->arch.hvm.tm_lock);
+    read_unlock(&pt->vcpu->domain->arch.hvm.pl_time->pt_migrate);
 }
 
 static void pt_process_missed_ticks(struct periodic_time *pt)
@@ -543,8 +554,10 @@ void create_periodic_time(
     pt->cb = cb;
     pt->priv = data;
 
+    pt_vcpu_lock(v);
     pt->on_list = 1;
     list_add(&pt->list, &v->arch.hvm.tm_list);
+    pt_vcpu_unlock(v);
 
     init_timer(&pt->timer, pt_timer_fn, pt, v->processor);
     set_timer(&pt->timer, pt->scheduled);
@@ -580,13 +593,26 @@ static void pt_adjust_vcpu(struct periodic_time *pt, struct vcpu *v)
         return;
 
     write_lock(&pt->vcpu->domain->arch.hvm.pl_time->pt_migrate);
+
+    if ( pt->vcpu == v )
+        goto out;
+
+    pt_vcpu_lock(pt->vcpu);
+    if ( pt->on_list )
+        list_del(&pt->list);
+    pt_vcpu_unlock(pt->vcpu);
+
     pt->vcpu = v;
+
+    pt_vcpu_lock(v);
     if ( pt->on_list )
     {
-        list_del(&pt->list);
         list_add(&pt->list, &v->arch.hvm.tm_list);
         migrate_timer(&pt->timer, v->processor);
     }
+    pt_vcpu_unlock(v);
+
+ out:
     write_unlock(&pt->vcpu->domain->arch.hvm.pl_time->pt_migrate);
 }
 
diff --git a/xen/include/asm-x86/hvm/vpt.h b/xen/include/asm-x86/hvm/vpt.h
index 39d26cbda496..74c0cedd11cc 100644
--- a/xen/include/asm-x86/hvm/vpt.h
+++ b/xen/include/asm-x86/hvm/vpt.h
@@ -128,12 +128,18 @@ struct pl_time {    /* platform time */
     struct RTCState  vrtc;
     struct HPETState vhpet;
     struct PMTState  vpmt;
-    /*
-     * rwlock to prevent periodic_time vCPU migration. Take the lock in read
-     * mode in order to prevent the vcpu field of periodic_time from changing.
-     * Lock must be taken in write mode when changes to the vcpu field are
-     * performed, as it allows exclusive access to all the timers of a domain.
-     */
+     /*
+      * Functions which want to modify the vcpu field of the vpt need
+      * to hold the global lock (pt_migrate) in write mode together
+      * with the per-vcpu locks of the lists being modified. Functions
+      * that want to lock a periodic_timer that's possibly on a
+      * different vCPU list need to take the lock in read mode first in
+      * order to prevent the vcpu field of periodic_timer from
+      * changing.
+      *
+      * Note that two vcpu locks cannot be held at the same time to
+      * avoid a deadlock.
+      */
     rwlock_t pt_migrate;
     /* guest_time = Xen sys time + stime_offset */
     int64_t stime_offset;
