diff --git a/pci/src/vfio_user.rs b/pci/src/vfio_user.rs
index a4ca1d484..8fd7c5f52 100644
--- a/pci/src/vfio_user.rs
+++ b/pci/src/vfio_user.rs
@@ -385,6 +385,7 @@ impl VfioUserPciDevice {
 
                 mmio_region.mem_slot = Some(slot);
                 mmio_region.host_addr = Some(host_addr as u64);
+                mmio_region.mmap_size = Some(mmio_region.length as usize);
             }
         }