diff --git a/xen/arch/x86/hvm/viridian/time.c b/xen/arch/x86/hvm/viridian/time.c
index ca6d526f46..9311858d63 100644
--- a/xen/arch/x86/hvm/viridian/time.c
+++ b/xen/arch/x86/hvm/viridian/time.c
@@ -108,8 +108,10 @@ static void time_ref_count_thaw(const struct domain *d)
 
     trc->off = (int64_t)trc->val - trc_val(d, 0);
 
+    spin_lock(&vd->lock);
     if ( vd->reference_tsc.msr.enabled )
         update_reference_tsc(d, false);
+    spin_unlock(&vd->lock);
 }
 
 static uint64_t time_ref_count(const struct domain *d)
@@ -331,6 +333,7 @@ int viridian_time_wrmsr(struct vcpu *v, uint32_t idx, uint64_t val)
         if ( !(viridian_feature_mask(d) & HVMPV_reference_tsc) )
             return X86EMUL_EXCEPTION;
 
+        spin_lock(&vd->lock);
         viridian_unmap_guest_page(&vd->reference_tsc);
         vd->reference_tsc.msr.raw = val;
         viridian_dump_guest_page(v, "REFERENCE_TSC", &vd->reference_tsc);
@@ -339,6 +342,7 @@ int viridian_time_wrmsr(struct vcpu *v, uint32_t idx, uint64_t val)
             viridian_map_guest_page(d, &vd->reference_tsc);
             update_reference_tsc(d, true);
         }
+        spin_unlock(&vd->lock);
         break;
 
     case HV_X64_MSR_TIME_REF_COUNT:
diff --git a/xen/arch/x86/hvm/viridian/viridian.c b/xen/arch/x86/hvm/viridian/viridian.c
index 7ea6c90168..c0be24bd22 100644
--- a/xen/arch/x86/hvm/viridian/viridian.c
+++ b/xen/arch/x86/hvm/viridian/viridian.c
@@ -494,6 +494,8 @@ int viridian_domain_init(struct domain *d)
     if ( !d->arch.hvm.viridian )
         return -ENOMEM;
 
+    spin_lock_init(&d->arch.hvm.viridian->lock);
+
     rc = viridian_synic_domain_init(d);
     if ( rc )
         goto fail;
diff --git a/xen/arch/x86/include/asm/hvm/viridian.h b/xen/arch/x86/include/asm/hvm/viridian.h
index 4c8ff6e80b..47c9d13841 100644
--- a/xen/arch/x86/include/asm/hvm/viridian.h
+++ b/xen/arch/x86/include/asm/hvm/viridian.h
@@ -71,6 +71,7 @@ struct viridian_domain
     DECLARE_BITMAP(hypercall_flags, _HCALL_nr);
     struct viridian_time_ref_count time_ref_count;
     struct viridian_page reference_tsc;
+    spinlock_t lock;
 };
 
 void cpuid_viridian_leaves(const struct vcpu *v, uint32_t leaf,