diff --git a/xen/drivers/passthrough/vtd/iommu.c b/xen/drivers/passthrough/vtd/iommu.c
index 234a4fbae5..68f9a524b8 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -142,8 +142,14 @@ static void cleanup_domid_map(struct domain *domain, struct vtd_iommu *iommu)
 
     if ( iommu_domid >= 0 )
     {
+        /*
+         * Update domid_map[] /before/ domid_bitmap[] to avoid a race with
+         * context_set_domain_id(), setting the slot to DOMID_INVALID for
+         * ->domid_map[] reads to produce a suitable value while the bit is
+         * still set.
+         */
+        iommu->domid_map[iommu_domid] = DOMID_INVALID;
         clear_bit(iommu_domid, iommu->domid_bitmap);
-        iommu->domid_map[iommu_domid] = 0;
     }
 }